1 fat quarter (or just some scraps)
Needle and thread
Step 1: Make a heart pattern out of some scrap fabric. Place in on the wrong side of your fabric (at least 1/4" away from the edges) and trace it with a fabric pencil.
Cut it out with a 1/4" boarder around the lines you drew.
Make some slits around the edge of your heart, cutting near the pencil line, but not right up to it. Repeat for as many hearts as you want to sew on your onesie.
Step 4: Now you're going to fold over the front of your heart about 1/4" so that the fold matches up with the pencil line. Take your needle and go directly down from where the thread is poking out of the heart and take a small bite out of the onesie fabric.
Tie it off on the back and admire your work.
That wasn't so bad, and this heart looks pretty darn good! Now repeat with your remaining hearts.
Caution: Doing applique in the dark while watching The Biggest Loser may result in whompy hearts like number 3 here. I'm too lazy to pick it out and redo it, so Ada will just have to deal.