Yes you work them down flat by one of several means. You can do it by hand by rasping, or you can use a jointer if you are careful about it and use paddles. The method I use, is to cut it down near flat with a bandsaw, then flatten it and thin it with a belt sander. You want the backing to be only about 1/8" to 3/16" thick at the crown, nearly a knife edge on the edges. If possible you want it slightly thinner as you go towards the tips too, since the limb will be thinner out there and you don't want to wind up wioth mostly bamboo. It helps to flatten, then draw your pattern adn cut it out so you aren't thinning a lot of extra material. Plus if the backing is to shape, where the width tapers you can get it properly thin.
