Here's a short tutorial. First, find cheap onesies. I purchased mine through a mom bargain site so they were super cheap, but a bit higher quality than just the Gerber brand you can buy at the craft store. Also purchase waste canvas. You can find this at the craft store.
Hoop your onesie and waste canvas. Now you are ready to embroider. I actually used a small cross stitch pattern that came on the back of the waste canvas package. I didn't want anything too large, but you could definitely do more than I did.
I think it's nice to add a little personal touch. Especially to something that ought to get a lot of use.
Once you are finished with your design you can take the hoop off, cut off the excess canvas and start pulling out the strings. I ended up grabbing a pair of tweezers to help pull the final strings out.
That's it! Now you can wash your little onesie and package it up with the rest of your gift. :)
There you have it. Have a Happy Day!