To make the sewing easier you need to tack them down with a nail. Do one side first and then place a strap underneath, tack it down and sew it. When sewing make sure that you always begin from the same side. This will ensure that sewing will be the same on all 8 stitching line.
The total length of the thread needs to be 45 cm.
Position the top strap assembly so it is centered. The strip of leather it to avoid scratches.
You see that the thread lay nice and parallel.