Have fun with it first and foremost ......but a couple of tips is to grip the fork before cutting to make sure it feels good in the hand, and two, always cut your forks and handle longer than what you need, to allow for any cracking while drying.
I always cut my handle especially long, as that gives me a good grip to hang onto while I am skinning the bark off and doing any carving......and POST pics please.
Take your time in finding just the right one. will be easier than you think, it will almost be like it found you. For your 1st slingshot I would find an already dry one because the drying techniques are a little intense. While your in your hunt for the forks you could cut a fresh one and it could be drying for later. there is a post page with detailed instructions on drying wood. Find a few so you have choices, or for the next build. There will me plenty more.