My first thought is to fuse the front to a cotton square and then applique it. For the quilting, I would like to maintain the look of the onesy, so I might echo stitch inside the edge, in someway highlight whatever design is on the front of the onesy(if applicable) and then do the same type of stitching to each one on the legs and arms (maybe a bit of a diamond or a flower). This would tie them together and hold them but not overwhelm the onesy. Then, quilt whatever you prefer in the background, sashing and borders. Please make sure and post your project.