This technique can be used for leg openings or any closed, round opening (neck, ankle, wrist...).
Overlap the elastic ends and stitch. We like sewing a little square like this. Then, fold the elastic in half (at the seam) to mark the half and join the half marks to find quarters.
Your elastic is now divided into fourths (seam + 3 marks). Now divide your leg opening into fourths, starting at the side seam (find the half first, then join the marks to find quarters).
Pin the elastic to the wrong side of the opening, matching marks. The elastic seam (0) should be at the half (1/2) mark of the opening (crotch lining). 4 pins, that's it!
Sew a first zigzag along the EDGE of the elastic (not down the middle). Always "zig" on the elastic, "zag" in the air (off the fabric). This creates a much cleaner look, with no raw edge showing. You will have to stretch the elastic between the pins so it lays flat against the fabric. DO NOT stretch the fabric.
Fold the elastic to the wrong side (the edge of the elastic should be against the crease) and sew with another zigzag or topstitch with your cover stitch. Stretch the elastic so the fabric is nice and flat (not stretched out) as you sew.
Easy, isn't it? If you are still not 100% confident, take a look at our video tutorial "How to Sew Elastics" on YouTube.