Made more for use than an instructable, I didn't take pictures along the way. It's pretty self-explanatory, but the most important things to do are measure to fit forks and start with the disk size.
4" diameter is enough to hold all my tools that I need to tear apart the motorcycle to bare frame and registration information or a few sodas if I'm out on a date. A fork bag is one of the best "small item" storage things you can put on a bike. I added a handle so that I can take it off when I need to work on a bike or if my motorcycle isn't in a secure area.
Tools: hammer, hole punch, drill, small anvil, razor knife and straight edge (i use a steel ard stick) some non permanent way to mark leather (Sharpies not a good idea unless you're using black leather)
Materials: 3 foot square of leather (the thicker the better), 4 buckles, and a 50 pack of hammerset rivets