A good sharp hatchet works well if you choke up on it and take accurate little chops. Be sure to start at the handle and chop toward the tips, if you chop toward the handle, you'll get into the grain and it'll split out on you. Thickness varies from stave to stave, but you can take the belly down to something like 3/4" at the fades, 5/8" at midlimb, and 1/2" at the tips. If it still feels like a 2x4 and doesn't bend at all, try 5/8", 1/2", and 3/8". Just take it slow and easy, especially until you have a few bows under your belt and get the feel of what you can get away with and what you can't. You can scrape and scrape with seemingly no change, then you get impatient and take some wood off, and you suddenly have a 20# bow. It's not easy to put wood back on once it's gone. I bet that cedar elm will make a good bow.