First thing you will need to do is aquire some onesies. They can be ones you already have, you can buy them in packs of 4 from Walmart for cheap, or you can buy some better quality ones from Hobby Lobby (which come in wider array of colors and prints). Once you have your onesie purchased, you will need some fabric (amount will depend on size of onesie) and you will need ribbon that is an inch and a half thick (enough to wrap around completely around the onesie with a little extra).
Now you are not cutting the bottom of the onesie off, you are simply sewing this “skirt” onto the onesie. Sew your rectangle into a closed tube. It should look like a cylinder with the print on the inside (kind of like rolling a sheet of paper into a long tube). Next you want to fold the tube up over itself so that the print of the fabric is now on the inside of the tube and the outside. The tube should now be half the length it was. The bottom of your double sided tube should be the folded side, and the top should be open with two layers of fabric. You can cut this top edge of the skirt to change how long the skirt will be on the onesie. I usually make it long enough to go about an inch or two past where the onesie ends.
The next thing you want to do is pin your skirt all the way around the onesie. This is why the circumference of your skirt didnt really matter because you can pleat it around the skirt so it fits exactly. Just make little folds in the fabric as you go and try to make them even all the way around. Once the skirt is pinned on, sew around the circumference of the skirt to secure it to the onesie. Make sure you are not sewing straight across with the onesie folded, or you will be sewing your onesie closed.
Once this is done, youll want to add some ribbon to hide the cut edges at the top of the fabric skirt. Start in the back with one end of the ribbon and sew your way around the skirt, ending in the back where you started. Once you sew all the way back to your starting point of the ribbon, just fold the end of the ribbon under to get a clean line (that way the ribbon won’t run). You obviously don’t want to sew right in the middle of your ribbon, so I sew two lines, one at the top of the ribbon and one at the bottom. This hides all of the skirt stitching securely inside, and underneath, the ribbon.
You can emblish the side with a bow or fabric/ribbon flower! The possibilities are endless. I made one of these onesie dresses for my niece to wear to church. I even used fabric paint to add a little faux peal necklace onto the top of the onesie.