The Leah leggings are not designed to have elastic in the waistband, but if you prefer to add elastic, I will show you how to change the assembly of the waistband to add elastic in one go.
To add elastic you will need to cut the front waistband in two pieces, so you will cut the front outer waistband and the front inner waistband.
This tutorial is substituting step 7-9 in the sewing instructions.
7. INNER WAISTBAND
Pin away the stitched pocket opening to make sure it doesn't get caught when sewing step 9.
Place the back inner waistband and the front inner waistband right sides together. Sew the side seams using a narrow zig zag stitch. Press the seams open.
8. OUTER WAISTBAND
Place the back outer waistband and the front outer waistband right sides together. Sew the side seams using a narrow zig zag stitch. Clip a notch at the top of the waistband, making sure you cut to but not through the stitches. Press seams open.
9. INNER AND OUTER WAISTBAND
Turn the outer waistband right sides out.
Place the outer waistband inside the inner waistband, right sides together, front waistband pieces together and back waistband pieces together. Sew using a medium zig zag stitch.
Note: if you are using an overlocker, you may add elastic and sewing the seam at the same time (instead of doing it separately as illustrated here).
Decide how tight you want the elastic around your waist. The elastic should be shorter than the top of the waistband circumference. Add 2 cm (3/4") to the measurement to be able to sew the elastic together.
Quarter the elastic and the top of the waistband and match up the quarters. Stretch the elastic while sewing it to waistband.
Continue sewing from step 10 in the sewing instructions.