I explained it in the video. What ever you take away as your off shoulder, multiply it by 2 or (add the front and back together) plus stitching allowance, what ever you get is for one side ( front and back)
For instance, if you take away 5" as off shoulder, to calculate the strap for one side, multiply the 5" by 2 ( for the front and back) for one side, then add sewing/ stitching allowance (1") to it. In total it will be 11" for one side. So for both straps you will have 11" in two places which is 22". When you are done sewing with half inch, on each side, you should have your 5"×2"(10 ") length of strap for each strap. I hope this is detailed enough???