"Box Breathing" or "Square Breathing" is free and can be done any time and almost anywhere (not underwater, for example).

It's named that because four cycles of breath are given equal amount of time, like how the four sides of a square are the same size.

Breathe In for two counts
Hold for two counts
Exhale for two counts
Hold for two counts.

The "counts" can be fast or slow. And be gentle with the process, especially the "holds," during which the most calming occurs. When you "hold," don't obstruct the airway. Instead, just stop the inhalation (or exhalation). The magic is in the pause.

Best of luck!