Fixing or making a purse strap is simple using the right needle and a straight stitch. This sewing video tutorial will help you make or repair a strap that uses a D-ring.
This would be similar for repairing a D-ring belt as well. The great thing about D-rings is that they are relatively simple to use in making all sorts of usable items.
When learning how to make or sew a purse, start simply with quick handbag straps using D-rings. You can find them in the notions department of your favorite fabric or craft store.