Cold water
I read that cold water on your face can calm you down in a few seconds. Meanwhile most of the advice out there wants ten quiet minutes of meditation.
I'm not against meditation. Good for the folks who can do it. But when I'm really worked up, being told to sit still for ten minutes sounds like being told to hold my breath underwater. My mind isn't going anywhere quiet. It's pacing.
A splash at the kitchen sink, though. That I can manage. No app, no timer, no getting it right.
There's something to be said for help that meets you where you are instead of where you ought to be. A lot of advice is written for a calm person who wants to be calmer. The rest of us need something that works mid-boil.
Sometimes calm is a big practice you build over years. Sometimes it's just cold water and a dish towel.