To make the ribbon ruffle, you need to do a basting stitch with your sewing machine. Set your stitch width as wide as it will go and the stitch length a high as it will go. Sew at the top of the ribbon. If you have thin ribbon, it may start to gather after you sew it, this is OK. Once you've sewn the top of the ribbon, hold the bottom string at the end of the ribbon and slide the ribbon on the string so that it gathers into a ruffle. Adjust it to fit your onesie.
After all the rows are sewn onto the onesie, cut the feather boa to the length of the neckline. Sew it to the neckline of the onesie with a zigzag stitch. Don't worry about it getting matted to the onesie, you can fluff it after you're done sewing.
Then, tie a bow to put at the top of the tree. Hand stitch it in place. I added a few small stitches down the ends of the bow to make sure it didn't shift as she wore it.