This tutorial is good for adding ruffles to anything really, but in this case, I was adding ruffles to a diaper I was sewing. It would look really cute on the butt of a onsie, too!
Take your ribbon and cut it to be almost twice as wide as the area you're covering.
I used 5 strips of ribbon, all the same length.
Heat the ends of the ribbon to seal them using a lighter.
Using a loose stitch, sew right down the middle of the ribbon. When you cut your thread, leave about an inch at the end.
Pull the bottom piece of thread while pulling the ribbon in the opposite direction. This will make the ribbon scrunch up. Do this for all 5 pieces of ribbon, trying to make them all about the same length.
Pin your ribbon to the butt of the onsie.
You'll want to switch your machine to a tighter stitch for sewing on the ribbon.
Sew on each piece of ruffled ribbon, right on tip of the original stitch. You'll need to lift up the foot of the machine to straighten the ruffle every few centimeters.
Trim your spare strings, and you're done!