It's pretty straight forward. I usually take one square foot of leather and lay it on top of the gun so that it drapes over the lock (at half-cock, frizzen closed). I move it back and forth until it "hangs" well and wraps over the stock with minimal gaps. This usually ends up being slightly forward of center. I then mark the leather from the "peak" down with a scratch awl, leaving an inch or so gap between the lines down each side at the top. This will form a hinge. Cut along each scribed line and tuck one edge under the other (doesn't matter which, but if there is a difference choose the way that gives a more horizontal outside edge so the rain will run down instead of getting a chance to seep in). Lightly mark where the flap line lays. Remove the leather and stitch so that the flap ends up where you marked it.
I then melt beeswax and smear it inside & out with a clean wood paint paddle (or any flat, pliable spatula). A heat gun can help work it in throughly (don't cook the leather).
You can then add button(s) or laces to attach it to the gun securely. A fancy variation is to round the corners.
***snipped the pics out**
Some folks lace them up tight as boots, but I like mine easy to remove. Just a strap long enough to tuck under itself holds well.
