You will need:
- Ribbon, 5/8 " width
- Ribbon, 1 1/2" width
- Seam ripper
- matching thread, scissors, etc...
Pin the heck out of it, and then sew close to the ribbon's finished edge:
Place and pin them UNDER the ribbon edge, taking care that the sleeves are even with each other. For the beginning and end of the sleeves, you'll want a nice curved edge. This is easy to do. At the start and finish (the cut ends) of the ribbon/sleeve's finished edge, move the finished edge of the 1 1/2" ribbon so that it meets with the 5/8" and makes a nice curved shape. I hope that made sense! You should have a little triangle of ribbon/sleeve hanging over in the inside, cut it off after you sew the sleeve on:
Also, since you cut that little triangle shaped piece off from when you made the sleeve curved, it left a raw edge on the ribbon. Raw edges on ribbon fray like crazy. Take a lighter and seal the raw edges of the ribbon; it's a good idea to do that on the ribbon in the underarm as well. Just don't set the onesie on fire, or yourself, or your house :). Sometimes it can get scratchy if you melt too much of the ribbon, so another alternative is using Fray Check.
There you have it! Now decorate the onesie till your heart is content. I added ruffles on mine: