The thick woven straps are a simple flat braid. I drew you a pretty picture:
Basically, you start at the left, and simply move each strand over-under-over til you reach the far side. This is exactly the same principle as a three-strand plait, if you think about it! It looks like they have used flat strips of leather (rather than round). I don't think it matters if you use an odd or even number of strands, as long as you always start with the same movement (either over, or under). And the tension that you put on the strands will pull it straight, you won't end up with a diagonal like my illustration!