First I added Heat 'n Bond to the wrong side of the fabric and cut out any shape, in this case I used a tie. Then, just iron on the tie to the onesie and sew around the edges. I thought I would have to do that by hand at first, but I was able to do it with the machine with some trickery.
If they weren't all over Etsy, I would think of selling them. By the way, I am obsessed with Etsy now. If you are not familiar with it I suggest you browse sometime. It provides a way for people to sell handmade or vintage items online if they don't have the time or money to sell them in a store. Brilliant.