Consider the back panel the main body of the bag. The back panel is stitched to the back strap that lays across the back of whatever you're riding on (horse/bike/unfortunate younger brother). Back panels can be any shape and size to meet the specific needs of the rider, but any adjustment made to the back panel must be made to the gusset and front panel as well or things will not line up well when you go to put them together. There are templates you can buy from Tandy leather factory (and other places I am sure) that make it very easy to lay them out when you first get started. After you get a couple sets under your belt then you start tweaking the designs on your own and that's when things get really interesting. Start by using a template or a stencil. Use a good thick leather like a 8 oz - 12 oz veg tanned cow hide leather if you want to tool some designs on it. You can literally make saddle bags out of any material, but you use veg tanned when you want to carve into it. For the purpose of this Instructable I am talking about tooling. For thick leather like this is really helps to have a good sharp blade. It doesn't have to be anything fancy. A good utility knife from Lowes is the tool of choice in my kit. As long as you can swap the blades out when they dull (and they will dull...quickly) then you will be in good shape.

Transfer the template onto the smooth/grain side of the leather so you can make sure your surface is good and you are avoiding as many defects as possible. Once the templates are transferred then start cutting then out. If you are shaky then do a rough cut all the way around and separate the two back panels from the hide leaving yourself a 1 inch border around each. Once you're done with the rough cut then go back and do a final cut right on the line. hold the knife at a good 45 degree angle. Press one hand down on the leather to keep it from moving around on you, press the knife blade all the way through the leather until you feel a little pop, and start pulling the knife down the line . If you have trouble seeing the line of your template position a good light source behind you so that the panel is well illuminated. It really does help to see where you're going. Cut your way around the panel until it is trimmed completely and then repeat the same step for the other panel. Remember, aside from the back strap, everything you do is in duplicate for these bags.