Hi mate,
I am over in Yorkshire, UK. You have picked a good wood, I like Ash alot. Especially for flat bows.
Ash splits nice and easily, usually! Go ahead and split that sucker, take the bark off and don't damage the back of the bow. I would cut the wood to bow profile as much as you can, seal the ends with PVC glue and clamp it down to a form and let it dry. IF you cut the stave down to almost bow dimensions it will dry a lot quicker. Just clamp the drying wood to a thick form and seal the ends and it should not warp while it dries.
Just make sure the bow limbs are tapered and you will be ready for floor tiller and long string in no time once the wood is dry. I normally make my ash flatbows 1 - 1.5 inch thick at handle, 3/4 inch thick at mid limb and half inch thick at nock so there is an easy taper to work from once the wood dries. These are not finished dimensions they are just a round guide of what I use when roughing out a flat bow while it dries. You can use whatever you want, this works for me.
Ash is tension strong on the back so a flat bow design normally works well, I would go with 2 inch wide if its your first bow with a handle of 4" with 2" fades either side and make it 72" long if you have a big enough piece of wood. Building the Ash flatbow long and wide like this will significantly increase your chances of a nice fast bow as a beginner. Start easy and make the difficult bows later once you have spent time making mistakes.
Good luck with it mate,
Dean