For a clean finish on your brief or top of the bra cup, the zig-zag stitch allows the elastic to stretch as when used on stretch fabrics like jersey or mesh.
1. With the right side of the fabric facing up, place (pin) the elastic on the edge of the fabric with decorative side facing in.
2. Sew the elastic with a zig-zag stitch to secure the elastic into place. You want the stitch as close to the decorative edge as possible and the edge of the fabric not to poke out past the elastic otherwise you'll be trimming the fabric before the next step. Keep your groove of your foot in line with edge of the elastic to ensure a straight stitch.
3. Turn the elastic back on itself so the elastic is now underneath when the right side of the fabric is facing up. You should just be able to see the edge of the elastic peeking out from the fabric. Secure with second row of zig-zag, the zig-zag should be near the edge of elastic so the elastic doesn't flip up and the edge of the fabric doesn't poke out.