In addition to all the other suggestions here for help, I would also advise reading the "The Traditional Bowyer's Bible” series I, II, III & IV. In particular, pay attention to the chapters by Tim Baker on bow design. There is a ton of really good info out there on this stuff and it will require a lifetime of experience to process it. More details than I could possibly list here. Best thing to do is to start, shave some wood, make mistakes, break wood, and get hooked for the rest of your life! It's a blast and IMO the basis of traditional archery!
"I would not recommend a walnut bow shorter than 66" though. It's pretty weak in both compression and tension. It makes a nice, lighter weight bow at that length though".
I strongly agree with John's recommendation here, Black walnut would not be an optimum wood for this bow design.
Good luck and have fun!
Kris