I measure the circumference of the onesie and double it.
So the measurement for your fabric will be double the circumference of the onesie and 10” long.
Iron & sew a hem on one side of the long end of the fabric. (not the 10” side)
Cut a piece of elastic that’s as long as the circumference of the onesie. Opposite the hem i fold down a little of the fabric and attach the elastic with a zig zag stitch.
This is where the the waistband will shrink up to size to fit around the onesie
Usually I don’t measure and cut the elastic. I just sew the elastic on while it’s in one big strip then when I reach the end of the fabric I cut the threads and elastic at the same time. This way I will always have enough elastic.
Then take the two short ends and sew them together to form the skirt.
Fold the onesie in half (top to bottom) to determine the middle.
Then fold it lengthwise to find the center.
I like to mark the middle and the center with air erasable marking pen. This also helps me keep the skirt straight when I sew it onto the onesie………no I did not learn this lesson the hard and very crooked way.
Pin the skirt in place using the back center as the starting point. The skirt should be wrong side out and make sure that the skirt bottom is facing the top of the onesie.
After it is all pinned sew with a zig zag stitch. I make sure to go over the edge of the elastic facing the skirt bottom.
Flip the skirt back over and you are done. If I left anything out or anything doesn’t make sense please let me know. I’m happy to help.