The advice to get T.C. Albert's book is good advice. Madison Grant's book is a great reference for creating an authentic design, but the Albert book really covers how to build and has several great patterns.
I usually build my bags out of 2-3 oz. veg tanned leather with 3-5 oz veg tanned for the strap. If I use a brass buckle, I burn the varnish off with a torch and then steel wool and age it with cold blue.
I use waxed linien thread to stitch and pre-punch the holes with a 4-prong chisel. Fiebings leather dye, then neatsfoot, then Kiwi polish.
My local Tandy store is very helpful (and they give a discount for my Boy Scout projects). The economy grades of leather are fine for pouches because you can place your pieces around the defects. Bellies are a cheap source for strap material. If you and a couple buddies go in on the materials, you can build a number of bags from a single side. I recall about 8 coming from about 25 sq.ft. for my Cub Scouts when they built bags.
Here are drawings for a beavertail-flap bag I have made several times.
Good luck. It is fun.
And you stitch them inside out. A welt (narrow strip of leather, punched for stitching holes) between front and back of the bag protects the stitching.
Larry Luck
