1. Start by only observing
Next time you masturbate or have sex, change nothing. Your only job is to notice where your body braces as arousal builds. A systematic way to look: pick one direction — from the top of your head down, or from your feet up — and sweep through the whole body once, front and back, muscle group by muscle group; when you find a bracing spot, pause on it for a second and just note it. The pass is for finding your map once — afterwards, attention goes back to what's happening, to the sensations and your partner, not to keeping a running body checklist. Common spots are the lower belly, thighs, buttocks, and the pelvic floor, but tension also collects in the face, jaw, shoulders, arms, and calves — and everyone's map is different. (This scan and the common-spots list are practitioner heuristics, not scientifically validated procedures.)
The goal is to catch the tension while it's happening, not to fix anything yet. If you find you can't feel much of a rising arousal to observe — some medications and low-energy states flatten it — see the safety note below before pushing on.