My name is Joe, There are five ways to get to the starworld. The easiest way is in doughnut plains. If you've gotten the green switch at the very end there will be a tall wall of green blocks and a triangular block.
Run up the wall and u will find a key and a keyhole. This will lead u to doughnut secret 1. There are two ways to clear this level. One of them will lead u to doughnut secret house. When u get to the yellow door and a hole underneath, hit the P switch and run back to the door, u will see yellow blocks under the door. Instead of going through the yellow door, hit the block above the door and a vine will pop out. Climb the vine, and run quickly to the right, if the P switch music is still playing u will see a blue door; go through the blue door and fight the big boo by throwing blue blocks at him. If u defeat the level this way, it should lead u to a star that will take u to star road.
Prepare yourself for the Great Beyond Mario!!!-Bowser