After you become flaccid, pull the T-leg forward. "Accordion-pleat" the end of the tape (use about 1/4" pleats).

Attach your elastic strap, wrap it around your body (stretching as you go), and fasten free end to your boxers. Remember to not stretch too hard -- 16 to 18 ounces should be adequate. You should feel no pain, and get no sores or blisters.