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Create a chest strap. Attach two shoulder straps. Acquire a pair of shields. Attach the guards to the harness. Put on the harness before your other clothing.
Measure the distance between your armpits to your upper torso. Use a simple piece of string, a small-diameter rope, or a body strap. Cut two pieces of material to this length and see if they fit around your body. The chest strap should be leveled to your arm pit on your upper chest.  Once you find a comfortable fit, seal the ends with a knot to prevent unraveling. You can also use Velcro to attach the chest strap. Measure and cut out two thin strings that will go over your shoulders and attach to the chest strap to hold it in place. Take a measurement while the chest strap is attached, then remove the chest strap to attach the shoulder straps.  You can knot the straps in place or use velcro to secure the shoulder straps to the chest strap. Imagine the design of a bra to help construct this design. There are plenty of options for underarm guards or pads. You can either make your own pads or buy a pack of premade pads designed to attack the sweat. Use whatever product or design that fit your needs. Use a safety pin or tape to attach the underarm guards to the harness where it crosses under your armpits. The guards should fit snugly against your underarms without impeding circulation. The easiest way to put on the harness is to insert each arm as if pulling on a cut-off T-shirt. Put your head through the harness. Adjust the length of the shields as needed. Once you're satisfied with the placement of the guards and harness, put the rest of your clothing on.  Determine if you can see the harness while fully clothed by looking in a mirror. One way to hide the harness is by wearing an under shirt over the harness and then wear an additional shirt. Loose fit clothing will also help conceal the harness. Give yourself at least one week to test the comfort of the harness before trying a new technique.