There are so many ways... Starting for example with "box breathing" can be very powerful, to calm the nervous system.

To align and synchronize brain- and heart frequency you can start also by breathing while counting on 4 inhaling and exhaling on 4. Or when going deeper, you count on 4 inhaling, holding the breath counting on 4, exhaling counting on 4 and holding again breath counting on 4. You do this for as long as it feels good and you feel how you calm down, realign, centering, grounding...

There are so many breath masters out there. I went quite deep into breath work with different trainings. One of them was SOMA by Niraj Naik