Find the center line of your bow, draw out the outline off of the center line and cut it out. If you follow the grain, the center line won't be straight.That's OK! If possible, any knots that might end up in the limb try to keep them away from the edges of your bow. If the wood is solid around a knot, it should be OK. If the wood inside a knot is pithy, remove it without hurting the surrounding wood.
You don't have to worry about the twist until later, if at all. Check out Jawge's site for dealing with twists. The way your stave lines up in a shallow "S" the string should track across the handle and that is OK too. Pat