Hickory is a good wood to start with since the sapwood, the first layer under the bark, can be the back of your bow. You just need a clear (knot free) piece. If you have a 6' piece - great! Make a 68" longbow. Even 6" diam will work, if the grain is straight, it should split pretty straight. 1/4 the log, strip off the bark, then seal the ends and back with white glue or shellac, and let it dry. Best to dry it in a horizontal position, this helps prevent any weird twisting, but it might twist/warp anyway - nature of the beast. 1/4's from bigger logs won't twist/warp as much. If your piece isn/t thick enough, you can always add some handle material.
Oh, like LBH said - keep your feelers out for some osage.