Very easy. So first u must b in glitsville. and at one point Koopa K will tell you about a new sort of hot dog and will tell you to check it out. go out side and you will see an egg jumping around the hot dog stand. whack it with your hammer once or maybe twice? and it will go on the roof of the stand. go on the top of the watering hole and turn into a plane on the panel on top of the Rawk Hawk billboard and fly to the top of the stand and the egg will ask you something and you say yes and you have the egg but it is not hached.
Then you must battle the Armored Harriors and lose. Then you get back to the room and the egg has hached into a yoshi.

P.S. Use yoshi's Gulp twice to defeat the Armored Harriors