Attach a large safety pin to one end of the elastic. Slide the safety pin with the elastic attached inside the strap guide and push the fabric down on the safety pin until It comes out the other end, being careful not to let the elastic twist inside the strap.
Pin both ends and stitch across the short ends to secure the elastic in place.
Adjust the bunched up fabric evenly across the elastic. Pin once in the center to keep elastic distributed evenly.
Stretching the strap out as far as it will go run a1/4 inch seam through the fabric and elastic down both sides of the strap. Set aside.