I was looking at one of Cami's dresses a while back and thought "I can make that from a onsie!" between moving and life, I haven't had time or the right materials until today. It literally took me about 30 minutes to complete. I even took pictures along the way so I can remember how easy it was!
Start with a onsie and a coordinating old t-shirt or piece of knit. I used a shirt, which means I didn't have to sew a hem or side seams on the skirt.
Lay the onsie on top of the shirt with about an inch from the bottom of the onsie to the bottom of the shirt. Cut through both layers about 2 inches below the bottom of the sleeve of the onsie (or where ever you want the skirt to start.
Sew a gathering stitch around the skirt (t-shirt) section. I used sewed along the front and back separately because it makes gathering easier. The shirt I used was a size small and the skirt isn't very full. For a more full skirt, get a large or extra large t-shirt. Keep an eye out at those Walmart clearance sales where shirts are like $2.
Pull threads and gather until the skirt is as wide as your onsie. Turn the skirt right-side out and put the bottom of the onsie inside of it. Turn the top of the onsie inside-out and place it upside down (so right sides are together) on top of the skirt. Be sure that the front of the onsie top and bottoms line up.
Sew around all three layers. I used a zigzag stitch because of the stretchy knit fabric.
Flip it right side out and you're done! Stick it on your little girl with a pair of tights and you're ready to go! I like this style of dress because you don't have to worry about an undershirt or tummies sticking out.
Not that it really matters that I added a skirt, because my baby lays like this all the time anyway. Babies are so modest!