First measure the width of the onesie and double the size to get your waistband for width and I made my length by 4 inches. Mark a light pencil mark across the onesie.
Fold in half and sew. This will be your waist band. Now cut a rectangle of either same fabric or coordinating fabric. Use the length of your fabric (about 42" by however long you like your dress. I made my to cover the bottom of the onesie.
Baste stitch your rectancle. Pull the string to create a ruffle.
Pin rectangle to waist strip and sew. Attach dress skirt to onesie (pin on the line you marked)and sew. This is not a very thorough tutorial, so please visit www.love-2-create-blogspot.com for specific instructions. They have a great tutorial for this dress.