science

You can do mild vagus nerve stimulation with deep breathing, as others have noted, as well as any number of other activities, methods and/or devices.

But speaking solely for myself, I often start meditation with vagus nerve stimulation in its simplest form, that of taking a few deep breaths that are OUT longer than they are IN.

So if I breathe IN for a count of four, I might breathe OUT for a count of seven or eight. My post-covid lungs are not the strongest, but it's not a competition: you just breathe OUT longer than you breathe IN.

I do it two or three times to change speed and for me it has a deeply grounding, settling effect.

I'm sure others have far more formal procedures, but I can't be bothered: this simple one works for me.