Fold over the top of your skirt twice. The width of the fold should be slightly larger than the width of your elastic. Make sure it is wide enough for the elastic with two stitches on each side.
Stitch around both the top and the bottom of your fold. This is going to be the casing for your elastic.
On the back seem of your skirt unstitch a small hole in your elastic casing. Thread about 28 inches of elastic (more or less depending on your waist measurements- the elastic should fit snug around your waist) Hint: if you attach a large safety pin to the end of your elastic it will be easier to thread through the casing. Once the elastic is all the way through your casing stitch the two ends together and sew back up your small hole.