My favorite strap is a sewable leather strap - but sometimes you want a matchy matchy look; so my second favorite is a soft vinyl strap with a foam insert.
In this live sesh, I show you examples of making them, and explain why I make them this way.
I also show you a way to keep them from twisting IF you don't want the foam insert.
For straps like this I always cut 2 x desired end width. For example, the straps above are 2" wide-- so I cut 4" x WOF. Fold once, then sew both long edges. Treat the raw edges if they really show, most times they don't.
For fabric or SOFT VINYL straps, I cut 4 x desired end width. For example if my strap is going to be 1" wide at the end result, I will cut 4" width and fold long edges to meet in the center. Then I insert my 3/4" foam strip, then fold again so there is only 1 open long edge. Top stitch this closed, then flip over and top stitch the other side for a balanced look-- making sure to sew both sides in the same direction.
As always - if vinyl straps are giving you a problem, try a Teflon foot or a walking foot. You can always hammer with a rubber mallet any seams that are bulky, or press with an iron and pressing cloth.
There are SO MANY ways to make straps- it's worth experimenting and finding YOUR best way. However you do it, you are correct. Don't let anyone tell you it's the wrong way or that's not how it's done. If it works for you, it's ok!