Resources
Box breathing
Box breathing involves equal parts inhale, hold, exhale, hold, then repeat. Choose whatever time interval suits you. This particular video is 5 second intervals and goes for 10 minutes. There are plenty of other videos on YouTube with other time intervals and for different lengths of time if you prefer.
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.