The how to for me are a few things that I can do any time and any place, whenever needed. It's a redirection of focus/thought.

I have to begin it and maybe that is what you mean by the how to? I learned when I was having bouts of emotional distress frequently throughout the day, especially at work. I was getting tired of heading to the restroom to sit in the stall and try to regain composure. I began by staying put and focusing on what my hands were doing, the texture of what they were touching, the pressure of what they were grasping, if they were cool or warm.

If that was not enough, then I moved on to that walk to the restroom: I focused on the soles of my feet as I walked, feeling every part of the step, the movement of the contact of my sole and toes throughout the step.

You can also simply sit and take time to really feel all the pressure points of where you make contact with your chair - thighs, back, arms - and spend some time just noticing.

Soon, I was feeling better by the time I reached the restroom.
Eventually, with practice - even just practicing to practice and not in response to anything - shorter times of redirection were needed and I was able to recognize that I was, in that moment, okay in spite of seeming to be not okay. The seeming to be not okay was the fear redirecting me from reality.