Fold elastic in half and half again, mark the quarter sections with chalk or fabric marker. You will have a loop of elastic with four evenly spaced marks.
Turn pants inside out and center the seams of the rise and seat. Working on a flat surface, place pins or clips to mark each side.
Next, slip the elastic loop over pants at waist.
Fold fabric 3/8 inch (10 mm) over elastic and pin elastic to pants, matching upmarked segments of the sides and center front and back seams.
Place more pins, evenly spaced, gently stretching the pants and elastic.
Starting at the side, set machine to the highest stitch length and sew a basting stitch over the fold and through the elastic. Repeat for the back and front seams as well as the other side. Sew additional lines as needed. These short vertical lines of stitches will replace the pins and hold the elastic to the pants. The stitches will not be visible on the right side.
Remove pins. Fold elastic over a second time, approximately 3/4 inch (19 mm). The fabric will be scrunched because pants have a larger circumference. Place pins both vertically at the basting stitches, then gently stretch elastic, tuck folded fabric and position pins horizontally.
If desired, make a tag to indicate the back. Take a 2 inch (5 cm) length of ribbon or twill tape and fold in half. Position at back and tuck ends under fold. Pin in place.
Sew waistband using a wide zig zag stitch 1/8 inch (3 mm) from the first waistband fold. Sew through the folded fabric and elastic, pulling gently as you sew.