Using the joined end as a start point, fold the loop in half and mark 4 quarter with pins. With the bag the right way around, place the joined edge and the other half way point at the bottom seam line of the bag, pin in place 10cm away from the bag edge.
Make sure to only pin through 2 layers of fabric.
Continue pinning the strap loop to the bag, 10cm from the edge, until it’s 10cm away from the zip.
Check the strap isn’t twisted in any way.