Four equal counts, four sides of a square. Sit or stand upright, feet flat, one hand on your belly. Breathe all the way out before round one.
Slowing to roughly four breaths a minute, with a pause at each end, shifts you out of a fight-or-flight pattern. Heart rate settles, the shaking in your hands eases, and the counting gives your attention one simple job instead of whatever spun you up.
Military and emergency crews use this one - some call it tactical or combat breathing - because it needs no equipment, works standing in the dark, and nobody watching can tell you are doing it.
Four rounds is a reset. Eight to twelve is a session. If four counts feels like a strain, drop to three and stay there for a week - a pace you can hold calmly beats a longer one you have to fight. Only lengthen the count once the current one feels boring.
Ease off if you feel light-headed, tingly, or short of breath: shorten the holds, or just breathe normally for a minute. Skip the breath holds and talk to a doctor first if you are pregnant, or have a heart or lung condition, high blood pressure, or a history of fainting or seizures. Never practise this while driving or in water.