You'll need to purchase some industrial strength velcro from a home improvement store. Usually a pack comes with around 9 feet of each side, so you'll be able to create a few projects with one purchase. You'll also need fishing line, and obviously some scissors. You cannot use string or nylon because the velcro will begin to tear the threads apart. Fishing line works great because it's strong, can be tied in knots and it too is cheap and innocuous.
Cut a 7 inch strip of the hook sided velcro and peel off the tape on the back side to reveal the adhesive strip. Be sure to not let anything touch the adhesive as it's extremely tacky and will be difficult to remove.
You'll need to create a lattice of fishing line. This means one piece of line cut to 7 inches long. Then you'll want to cut a foot long piece of line and larks head knot it around the center of the 7 inch piece. This will be your anchor line. Finally create a series of short 1 inch lark head leads along the 7 inch line, these will help reinforce the lead line between the two pieces of velcro.
Now cut a 7 inch strip of the loop sided velcro and sandwich the fishing line lattice work between the two adhesive sides. Now you have a self locking velcro loop. Attach a metal ring to the end of the anchor line using a standard sport fishing knot like so:
You now have a device that can be wrapped around your erect penis. Adjust the diameter for a comfortable fit that won't slide off your erection. Choose either the loop side for a comfortable fit, or the hook side for a slightly painful side if you prefer.
Because the velcro is industrial strength, it's difficult to open the loop once sealed, especially when it's around your sensitive member. It's also difficult to open around your wrist with only one hand, so you can make a set of these for your wrists and ankles to make a good spread eagle setup.
Be sure to trim off any overlap between the two velcro pieces otherwise you'll have a painful time removing it from your penis.