I generally use 8-9 oz leather and have patterns for various sizes (i.e. caliber and quantity of balls). Most end up around 6 to 6 1/2" x 3 1/2". Since I normally make to order, I end up carving a plug out of cedar, sometimes holly (hate carving holly), walnut, oak or chestnut. Plugs are attached to the bag with either a brass chain or braided linen The first one I made in about 1977 was out of about 6 oz very soft leather and was turned inside out. I don't think it's worth the effort.
Over the years I have settled on making them as Hanshi described... draw from a pattern, cut, glue, hole the leather via punch or you could always drill the holes with a 7/32 bit, stitch, then like he said, soak in hot water, use a mandrel to open the pouch then fill with balls then gently slap each side to form the bag. Let dry and then apply your choice of leather conditioner. Oh, by this step, I've generally got a plug well on its way so I will start forming the opening to the plug, while the leather is wet.
I have some pictures, but I am a newby here and darned if I can figure out how to post pictures. Oh well....