The vintage project articles are good but they assume a nice piece of clean white wood (rock maple, hickory etc) doing something with a bit of knotty twisty half seaoned native wood is a different proposition and you need either some help with the first one or to fail a few times.
On the other hand a bit of hazel will make a bow that will last a bit before it doesnlt straigten any more. Two bits of hazel bound in the handle will also work for a while and you'll get a sort of tiller with the thin bits at each end.
Debarking and tempering the wood will make it more resiliant and more brittle.
For an ash bow that is going to end up about 50lbs or so:
Flatbows need a bit of wood about 2x1 the thin end will be 1/2" x 1/16" roughly and at the handle the wood will be about 1./2" with a flare to the full depth for the handle. The handle will be an inch across and an inch deep with fades to transition from the limb to the handle
Longbows need a bit of wood 1" x 1 1/4" tapered to half inch a square the the ends. The belly is then rounded off to a D cross section. No fades, no handle just a tapered pole.
Pyramid bows, possibly the easiest and can take the crappiest wood, can be up to 4" across at the handle tapering down to a point at with an even thickness of about 5/16". The handle will be an inch across and an inch deep.
All measurements are starting measurements you'll have to work with the wood to get an even bend.
The rule of thumb is don't breach any growth rings on the back because the grain will lift. The implication of this is that even if you are making a flat bow the back may be slightly dished outwards. Take wood off any bits that look thick and leave alone any elbows until you get a nice arc.
Pulling the bow up should be progressive with lots of pull up and let down cycles (reverses) with standing back and examining the bend at every stage.
I hope it's clear, it'd be much easier if you could see the sketches I've made on my notebook
There are some of my bows from start to finish on
http://groups.msn.com/bowsinprogress.