This one is a bit hard to explain. Find an area with a bunch of blocks and an item that you can carry. Try to kick the item into a block and hit your head on a block in rapid succession or simultaneously. Sometimes this will make a block appear out of nowhere! The type of block you can create depends on your location.

Ideal course: Chocolate Island 5. Pick up the P-Switch and use it as your item with the square of blocks above you. You can create a lot of Rotating Blocks here.

In Donut Plains 1, take the Key and use it as your item with the wall of Green [!] Blocks. You can create some Green [!] Blocks here.