Resources
Box breathing
Box breathing involves equal parts inhale, hold, exhale, hold, then repeat. Choose whatever time interval suits you. This video is 5 second intervals and runs for 10 minutes. There are plenty of time interval videos on YouTube including different lengths.
5.5 breaths per min
This practice involves equal inhales and exhales for 5.5 seconds each. To learn why this breathing style is so powerful, check out James Nestor's book, Breath.