There are 3 parts to making the staps, the main piece, attaching the buckles, and sewing the bottom of the adjustable part to the bottom of the bag.

My own measurements are tagged in the first photo, I based them off the length of straps on my other backpack keeping in mind that it was going to be adjustable anyway. The top 3" inches are folded over and sewn to the back. As for the buckles, I found an old purse laying around and took them off of that, this was convenient because the adjustable straps that went to the buckles originally were long enough to work with my backpack with minor alterations. To attach the buckles I could've made the rectangular piece on the bottom of the strap twice as long but I didn't think of that when I was planning so as you can see in the photos I had another piece wrapped around, which was only a problem because I had to punch and sew through 3 layers.

As for the bottom. (shown in the last photo) I had to sew through 3 layers again but the middle one (the strap stolen off the purse) was much thinner than my usual leather. First I sewed the piece that sandwiches the strap to the backpack, (it would be impossible if the steps were reversed), just by lining the parts up and punching the holes in the back the same as I had the rest. After that, it was just sewing the strap into the sandwich (parts shown in the second and third to last photos)

If I were to do this again I would find a better way to attach the bottom to the body of the bag, its been working fine but I worry about how long it will last.