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Step 1: Measure and Tape
First off, measure your waist. You want to make the belt as accurate as possibly so you don't have too much extra slack. Also measure your belt loops to see how wide to make the belt.
Get your duct tape and draw out a long piece. Lightly grasp close to the midpoint of the piece and fold one half on top of the other. It takes practice to get good at this, so you might have trouble the first few tries. See the picture below for a visual. Squeeze out all of the bubbles in the tape and continue on until you have enough to fit around your waist.