It's best to have a full supply of green eggs for this trick. Find a place in a level where you can aim straight up without hitting anything else (i.e. level 6-8 would be a good one, after you hit a door, but don't go in yet). Stock up on eggs if you don't have any, then aim straight up. Throw an egg and after it hits the ceiling once, have Yoshi catch it with his tongue. It should stop bouncing and be a yellow egg (if it was green to begin with). Throw another egg, this time letting it hit the ceiling twice, then catch it. It should have turned red. Doing this trick right will help you make your score better, as red eggs give 2 stars that can be used on the Baby Timer that appears when Mario falls off Yoshi's back.