Three techniques are used quite widely:
Cardiac coherence: breathing exercise at a regular pace.
The technique and its rationale is explained in the latter half of the following article
https://www.scientificamerican.com/article/proper-breathing-brings-better-health/
Video exercise:
https://vr-well.com/video/cardiac-coherence-exercise-607-mono-relax-6-10/
Jacobson’s relaxation technique: tension then relaxation of muscles
https://www.healthline.com/health/what-is-jacobson-relaxation-technique#qampa
Video exercise:
https://vr-well.com/video/jacobson-relaxation-1120-mono-relax-6-10/
Schultz’s autogenic training technique: concentration exercises on the parts of the body and associated sensations of heat.