Your friend probably has a strap now. Take it and on a large sheet of paper draw out your own pattern. Ask him what he liks or dislikes, do dimensions need changed?
Your first one may now be the best final design, but that makes a good start. I just finished a couple of camea straps and the next ones will be changed as some of the dimensions are not quite right. Now I have a reason to buy more leather!! I always make the first of anything out of a lower grade leather so if it gets filed in the trash it dosen't hurt the wallet as bad. Hopes this helps you get started. You can also make a paper mock up to check dimensions.