I was fortunate (or lucky) enough to totally complete this game without a strategy guide, so I know how frustrating this game can be at times. If you're stuck in some level, I hope this guide can help.
To get 100% in a level, you must finish with 20 Red Coins, five Flowers, and 30 seconds remaining on the countdown timer. This guide will focus on finding Red Coins and Flowers, and will not focus as much attention on collecting Stars to refill the timer, mainly because finishing a level with 30 on the timer is simply a matter of passing through Middle Rings, collecting Stars from Winged Clouds, and avoiding damage as you near the goal. There are also items available in bonus games that, when used from the pause screen, add 10 or 20 seconds onto your countdown timer. Keep a few of these in your inventory; they come in handy when you've reached a goal and you notice you have less than 30 Stars.
Once you have 100% for stages 1 through 8 of a World, an "Extra" and "Bonus" stage will replace the question mark areas on the map screen. Once unlocked, the Bonus stages will always be there for Yoshis in serious need of items! Oh, and consider yourself a "pro" if you can get 100% in every Extra world!
This one's not too tough. Make sure you go down the first pipe, and leave no Winged Cloud un-egged.
When Yoshi morphs into a helicopter, stay up high and check all the walls for Flowers and Red Coins. It may take a few tries to get everything, but you can fly around as many times as you need to.
Pound the second post for a Red Coin, and when you come across a group of three posts, pound the middle and right-most post all the way down for one Red Coin each. Near the beginning, two of the Flowers are tucked away in low pathways.
At the beginning part, there are a ton of coins above the ceiling. Just egg all the cracked blocks in the ceiling, then jump on top and run around... but don't forget to go back down and get all the Flowers below. After you go down a pipe, you'll eventually find a beige teeter-totter thing. Stand on the right side to tilt it down towards the lava to get that Flower, then keep tilting it down as far as you can, and then rush to the raised left end and jump up for another Flower. A little bit past the locked door, you'll come to a dead end with a flower pot sitting on a platform above you. Push it off the platform, and it will hit the ground and break, revealing a key that was hidden inside. Take it back to the locked door, and inside you'll encounter another door. Load up on eggs, and enter. Inside, kill all four of the Piranha Plants, and a Flower will appear.
Burt the Bashful: Duck down in the low areas, and Burt will never be able to jump on you. When the coast is clear, jump out and pelt him with eggs. Repeat.
Welcome to auto-scroll land! Just be patient, and you should finish with an easy 100%.
As "Mole Tank" Yoshi, dig high to quickly find the two Red Coins.
Finding all the Red Coins in this area can be tricky. First, don't pass up the Flashing Eggs near the locked door! Grab them and shoot them at enemies to get a Red Coin from each Egg.
Near the Goal, pound the red and yellow bridges to break them. Swim and get all the coins there. Then go stand on the highest point on the ramp thing sticking out of the ground, and jump straight up. A Spring Ball will drop down. Spring yourself straight up to find a hidden area. In the hidden area, go left, stand on the right edge of the Chomp Rock to get it rolling to the right and it will reveal a hidden Winged Cloud. Shoot it with an egg, and it will grow a Beanstalk. Climb up the Beanstalk for four Red Coins.
The Key is in the flower pot again. Use strategic platform-flipping to access the flower pot and a few Red Coins.
Salvo the Slime: You can break him down to nothing with just a few eggs. Salvo can't hurt you, but he can push you into the lava. And the Lemon Drops he drops can also hurt, but they make great snacks for Yoshis with low ammunition.
After the Middle Ring, you'll find a Winged Cloud under a long wooden platform, being guarded by a Slugger. Shoot two eggs just over the baseball bat-wielding baddie's head to pop the Cloud and collect the Flower. Shortly thereafter, you'll come across a Green Glove guarding some Coins and a Winged Cloud. In order to collect the Red Coin down there, shoot an egg horizontally into the chamber from the left so the Green Glove can catch it, then get down on the ground by the red arrow on the other side of the wall. The Green Glove will heave the egg at you, but it will harmlessly fizzle down the pit after it collects the Red Coin for you.
When you come to the area with a ton of break-away wall stuff above you, there is a Switch hidden in the left side. Use a few eggs to find it, then stomp it and press Down where the blinking red arrow is pointing. This takes you to a secret room hiding a bunch of Red Coins.
There are two Flowers in this stage that are easy to miss. The first can be found in the second door you find, which leads to a room with two Big Boos. You cannot hurt the Boos when they are covering their faces, so turn your back to one and bounce an egg off the wall at it. Do the same for the second Boo, and when both are defeated you'll be rewarded with a Flower. The second easy-to-miss Flower is after the log ride across the lava. In the next room, find the cracked ceiling block and break it with an egg. Jump up on top of the ceiling and go left to find a bonus room. In the bonus room, shoot one of the Boo Balloons three times to pop it, and he will drop a Switch. Step on the switch and hurry over to the Flower in the upper-left corner of the room.
Bigger Boo: Just like the Big Boos, he is only vulnerable when your back is turned to him. Show him your back, and then bounce an egg off the wall at him. Repeat until victory is yours.
The locked door leads to a Flower! To get the Key without committing Yoshicide, push the Crate onto solid ground before pounding it. To find all the Red Coins in this stage, make sure you pound the posts...
This stage is tricky! Remember to explore every nook and cranny. And when you're on the ground floor of the cave, look up every once in a while to find some break-away walls... One patch hides a door, and inside you must kill all four Lantern Ghosts for a Flower and some Red Coins.
I'm sure you're used to this by now: pound, pound, pound for Red Coins.
There are two Keys hidden in the flower pots...
Roger the Potted Ghost: Don't let the Shy Guys push you off the edge! Whenever there is an opening, constantly push Roger to the right, and soon he and his Shy Guy helpers will be long gone.
Quickness and perfect timing is the key to surviving this fast-paced stage. Don't forget to collect Flowers and Red Coins, too. ;)
I'm sure you know the deal by now: Pound the posts for some Red Coins.
At the part with the Paddle Wheel lift that is partially hidden by a tree, go left and pass the Egg-Plant to find two Red Coins. Then get on the spinning wheel, and jump up and left into the tree. You will find a hidden area with four Red Coins and a balloon-toss mini-battle bonus game.
Prince Froggy: Eat the Shy Guys for eggs, and then throw them straight up at Froggy's uvula. After about 10 hits, Froggy's had enough and spits you out.
After the part with tons of vines, you'll come to an area with tons of tall, skinny trees. Immediately go down and left to find an Egg Block, and then go down and left again to find a bonus area containing a Flower.
When you start this stage, use the third or fourth tree to leap to the sixth treetop. From there, jump to the right to find four Red Coins along with a second entrance to the cave. This high entrance is the only way to find the Key and one Flower.
The second Beanstalk you sprout leads up into the foliage. Get on the top of the Beanstalk and jump to the right to find a hidden area that has—you guessed it—Red Coins and a Flower.
Naval Piranha: When his bandage is exposed, shoot an egg off the wall so it skips across the water's surface and into his wound. Three hits puts Mr. Piranha down for the count.
The first room inside the fort has passageways shaped like an "X," with a door in each corner and a locked door in the center. Each corner's door holds a Key somewhere inside. Inside the locked door is three more locked doors, so don't bother opening the first locked door until you have four Keys.
Marching Milde: Just keep pounding her until she's a pile a Milde dust. The bite-size Mildes are also vulnerable to egg attacks.
When you find a beige block sitting on some spikes, pound it a couple times. This will flatten it out, allowing you to get the five Red Coins on the left. Oh, and whenever you find a Chomp Rock, push it as far as you can. Without it, you won't be able to reach all the Flowers.
From the start, go to the right and into the water. Hold Down as you hit the water, then during the brief second that Yoshi is completely underwater, swim left. You'll emerge on the other side of the wall, where you can find a Red Coin. Then in the second part of the cave, take the bottom route first, which leads to a room with about five Red Coins.
About one-fourth of the way through this stage you'll find a vertical line of three Coins. Shoot an egg lined up with the Coins, and a Spring Ball will fall at your feet. Hop off it to find a bonus area with tons of Coins. When you return from the bonus area, get the helicopter thing if you want, but hug the ground for five Red Coins and a Flower.
This stage is a toughie. Sooner or later you will come to a door with an "X" over it. There should be Koopa Troopas to your left, a Spinning Wheel arrow above you, and to your right, a passage heading downward. When you first come to this important area, have Yoshi grab a Koopa Troopa, and go down the passage. Spit the shell out at the Winged Cloud at the end of a narrow passage to your right. This will build a stairway to a door up and to your right. Go in that door, and you'll find a Crate with a Key in it. Then go through a door or two, and you should be back where you started. This time, go up and ride the Spinning Wheel across the spikes, then go in the door to your right. The rest of the way to Hookbill the Koopa is easy.
Good luck! Hint: Unless Yoshi is standing on something, he cannot enter tunnels that are a bit higher than the ground...
If an icicle is in your way, break it off with an egg. Other icicles fall when Yoshi gets close; keep your eyes open!
You can melt the big, clear ice blocks by eating a Red Melon and using your newly-acquired fire-breath on them. There aren't a lot of Red Melons, so use 'em only when you need 'em.
When you reach the beige teeter-totter, stay on the left end for a Red Coin, then drop down and go to the right, where you'll find a beige squishy block. Push it to the left to help you reach the Flower, then push it to the right to exit.
Sluggy the Unshaven: Aim at his heart with every egg shot. Eventually you'll make a big enough depression in his body to get one good shot at his heart. Hit Sluggy there two more times to defeat him.
Lots of the Red Coins in this level are in plain sight, but some are tricky... After the green Paddle Wheel falls down, you'll have to jump onto a yellow platform. Ride that for approximately 2 seconds, and when you see a Raven, immediately jump over to the red platform. That will send you up and to the left where you'll get two Red Coins easily, and it'll take you back to that yellow platform. When you go down the first pipe, use an egg to break away the blue wall for three more Red Coins behind it. Then, when you get on the dropping platform left of the first Bullet Bill, ride that all the way down for two more Red Coins, but quickly jump off and to the right so you don't fall down too far and die.
Some of the Flowers and Red Coins can only be reached by jumping off of Bullet Bills.
Raphael the Raven: The battle against Raphael takes place on a small, distant planet. While avoiding Raphael and his fireballs, try to get on top of a post. A well-timed pound will push the other end of the post through the other side of the planet and into Mr. Raven. Smack him with the posts three times and he'll be seeing stars!
After you kill the slime thing for a Key, you'll enter a multi-level room with vertical pipes providing transportation to the other levels. Once you enter, go to the right until you find a pipe leading down. Go down that and then go all the way to the left and into the pipe with two arrows pointing at it. Beat the slime in there for another Key. If you run out of eggs before you can beat him, exit the room and go down the pipe right outside the room, then left to find a room with six eggs, then go back and try again. Once you've gotten the Key, check out both ends of each level to get all the Red Coins and Flowers, then unlock the corked pipe and proceed.
Tap-Tap the Red Nose: Break the red/yellow/green blocks on the ground to create a lava pit, then try to push Tap-Tap in the lava with eggs. If you're lucky, Tap-Tap will hop into the lava by himself.
Each of the four posts at the end of the stage hides a Red Coin. Pound 'em!
At the beginning, get in front of the second Chomp Rock and push it to your right, until it lands in the little hole. Then jump off of Chomp Rock onto the platform above, where you'll find a Crate and two Red Coins. When you find another Chomp Rock sitting above you, break the soft ceiling to free the rock, then push it to the right. Keep pushing, and eventually you'll find the Key. Grab it and go down and through the bottom corridor, where you'll encounter a Spring Ball at the end. Spring all the way up, and then go back to the locked door. In the next area, the bottom gets confusing. Parts of the area are numbered 2 - 6, and entering a door takes you to the "other side." You'll have to fully explore both the dark and orange sides to get a 100%. On the orange side, when you find yourself between numbers 6 and 3, shoot an egg straight up to release a Spring Ball. Spring up as far as you can for a Flower and two Red Coins. On the dark side, left of the number 2, above the door you'll find two Red Coins, and a Winged Cloud containing a Flower. You'll have to bank two eggs up the skinny tunnels -- one to pop the Winged Cloud, and the other to collect the Flower. This one may take you a while.
About halfway through, you'll encounter four doors spinning above you. Shoot one down to choose your path. Door 4 leads to the easiest path, so try to drop that one if you can.
King Bowser: Bowser's first attack phase is ground-pounding, which sends shockwaves rolling through the ground. To keep Mario on your back, jump over the rolling ground, and then counter-attack with a ground-pound of your own. When Bowser eats ground three times, Kamek returns and rapidly enlarges Bowser, which sends Yoshi, Mario, and Bowser outside the castle. Dodge the falling rocks, gather some eggs, and launch them at Bowser. Keep hitting Bowser in his open mouth, and soon he will taste defeat.