video Mending Jeans Belt Loop
How to mend the belt loops of jeans or other pants. I used an iron-on tape on the inside of the jeans; snipped a bit of denim from the hem; repaired the torn out area from the back of the pants and then sewed the belt loop back in place.

If you don't have a darning foot, you can still do this by sewing straight stitches back and forth across the patched area, then turning the fabric 90 degrees and sewing some more straight stitches. The idea is to "sew in" replacement fabric where the hole is and then sew on the belt loop.