Cut your material slightly larger that the waist band. About 5 mm (0.25 inch) on all sides.
Fold it's sides under so that the edges are hiden.
Stitch around through the actual stitches on 3 sides. The stitching will get hiden in plain view onto the original stitching. Make sure that it's colour matches, or is slightly darker than, the original stitching.
For the last side, use a "spiral" stitch where the needle goes in next to the patch and out through it. Not eastethic, but safely hiden from view.
You may want to have double thickness to get it stronger or stiffer. You may attach the bar or hook beforehand.
Electro
May 2016