The protocol exists because judgement is the thing that has failed. It should be short, physical, and specific enough to execute badly and still work.
A workable one: flatten or verify all stops, cancel resting orders, stand up and leave the desk for ten minutes, breathe slowly, write two sentences about what happened, then re-read the plan. Trading resumes only at reduced size, and only after a fixed interval, or not at all that day.
Two details decide whether it gets used. It must be written down and visible, and it must include a step that physically separates you from the platform, because a protocol you can complete while still clicking will be completed while still clicking. See personal-circuit-breaker.
Related: tilt, tilt-triggers, personal-circuit-breaker, implementation-intention