Set your sewing machine to a zig zag stitch, a small stitch length and a small tension, as recommended in the video.
Stretch the fabric to establish which way it curls, towards the right or wrong side.
With the presser foot up, place the fabric under the foot with the curled side on top. Pull the raw edge of the fabric both front and back to form a roll and only then lower the presser foot to catch the roll.
As you sew, keep pulling the fabric which will continue curling. Make sure you pull the fabric at the front AND at the back at the same time, trying to keep even pressure on each side.
Once you're done sewing, the fabric will regain its original shape while the lettuce edge hem is longer so it curls up and down in waves.