Cut 3 strips of fabric 3x15 inches.
Serge, zig zag, or cut with pinking shears each of the edges. This will keep the fabric from fraying after it's been washed. Note: You don't have to do this if you're using knit fabric.
Sew a gather stitch down the middle of each strip of fabric. This means that you set your tension to the highest setting and your seam length to the highest setting and sew a straight line. Leave a lot of thread on each end.
Pull the top thread (the non-bobbin thread). This will cause the fabric to gather. Just keep pulling and adjusting your ruffle as you go.
See how it is gathering up so cute?
All three fabric strips are gathered and ready to be sewn onto the onesie.
Sew the strips onto the back of the onesie. I tried to place them really close together so that it would make the ruffles stick out more.
How precious is that?