You will need
Jeans needle (optional)
The goal is to make two slits one to two centimeters away from the belt loop and sew the elastic into place underneath the belt loop.
Cut two slits in the waistband with your seam ripper. Make sure that you only cut on the inside of the waistband. Before you start with making the slits check the other side of the waistband.
Check if the elastic fits in the hole if not make the hole bigger then cut out the elastic that is shorter than the distance between the two holes.
Attach the safety pin to the elastic then loop the elastic through the other side of the waistband.
If you want use fray check on the edge of the elastic.
Secure the elastic on both sides with a pin on the waistband and then sew the elastic into place with a couple back stitches. If you can sew it underneath the belt loop otherwise close as possible.
(If you want a cleaner finish you can push the elastic inside the hole and then sew the elastic into place.)
This is how it should look like once your finished.