A lettuce hem only works on stretch fabrics. The hem should be added to the direction of the fabric with the most stretch.
Lettuce hem with a sewing machine
Set your sewing machine to a short zigzag stitch. Fold the hem right sides together by 1/4" (0.5cm).
Stitch over the fold with the zig-zag stitch while stretching the fabric. The more you stretch, the wavier the edge.
Lettuce hem with an overlocker
Set your machine to a fine rolled hem and put the needle in the right position. Stretch the fabric while feeding it through the overlocker. The more you stretch, the wavier the edge.
Sewing Machine: https://www.youtube.com/watch?v=D5bH7xMxF_M