Cut your elastic to fit around your thigh (wherever you'll be wearing your garter). Subtract a few inches so that the garter will fit nice and snug.
Cut a strip of fabric that is 3 inches wide and double the length of your piece of elastic. Fold in half with right sides in and sew along the edge.
Turn the tube of fabric right sides out (I used tweezers). At this point, you'll want to add lace, ribbon, etc. to the tube- however you want to decorate it.
Use a safety pin to thread your elastic through the tube, making sure it stays flat. Sew together the ends, add some ribbon-y, button-y flair and whatnot, and you're all set!