First, this method works for any size or shape of saddlebag, whether its a slant, straight, big bagger style or the little single bobber bag I just did. You can either make the seams lap, like this one, or invert the seams by sewing the bag inside out and then turning it. The only thing with inverted seams is that turning them rightside out is a real muscle job and you need to soak the bag until it's soft.
This is the first saddlebag I've made with undyed leather. I've always used latigo, bridle or harness leather around 10oz
First, make your pattern. There are actually only 2 main pieces to figure out, - the front panel (same as back) and the flap. No need to make a pattern for the gussets. Then, there's small pieces like the weather flaps and buckle chafes. On a set of bags you'll make a yoke. If I'm making a set for a specific bike, I dont bother making the yoke in 2 pieces, becuse the size will never have to be adjusted. The yoke will vary depending on where the seat sits and the rear lights. I do yokes with either 2 layers of 4/5oz chrome tanned, or suede on the inside layer.
(1) OK, heres the pieces cut out
(2) Next, I stamped the little insert piece for the lid. That's just the design I thought of - usually the flap is just straight across and the straps are added after
(3) The lid dyed and assembled. I almost always line my small bags or just the lid on big bags with whatever upholstery or garment leather I have on hand. (note the very obvious stitching screw-up)