Breathing, a relaxing tool and suitable for various circumstances. For several years, one of my fathers gave me singing lessons. An important part of warming up your vocal instrument is a breathing tool. In addition, this way of breathing is also very helpful to use in case you feel overwhelmed, are panicked or in any other situation you feel you need to calm down. Studies have shown the positive effects (example1, example2). It could even help to fall asleep. Research and you will find many tutorials, articles, etc.
Stand still, or sit down.
If you feel like it, you can close your eyes.
Or, focus on a specific point in the space you’re at.
Breath-in for a duration of 4 seconds.
Hold that bit of air within, 4 seconds.
Breath-out for a duration of 4 seconds.
Don’t breathe for 4 seconds.
Count the seconds in your mind, it will help you focus on your breathing.
You can inhale through your nose, exhale via your mouth.
If you find it easier, start with also inhaling via your mouth.
You can variate in duration, the example above is what I am used to.
Experiment while you feel already calm and find out which way feels comfortable.
The best part; you can do it almost anywhere!