EDIT: for anyone curious, here is the official guide on Wim Hof breathing, copy-pasted from
https://www.wimhofmethod.com/breathing-exercises---
Never practice the breathing exercises in a place where it could be dangerous to faint.
Step 1: Get comfortable
Before you start practicing the breathing exercises, it is important to get as comfortable as possible. Sit in a comfortable posture in which you can expand your lungs freely. To make sure that you can expand your lung for the fullest, it is recommended to practice the exercises on an empty stomach.
Step 2: 30 power breaths
For this first breathing exercise, imagine that you are blowing up a balloon. While using your midriff fully, inhale through your nose and exhale through your mouth. Use short and powerful bursts. Close your eyes and repeat 30 times. You can become light-headed during this exercise or experience a tingling sensation. This is normal.
Step 3: Retention after exhalation
After your 30 power breaths, fill your lungs to maximum capacity, but don’t force it. Exhale and hold for as long as you can. Hold your breath without forcing it, until you experience the gasp reflex.
Step 4: Recovery breath
Inhale to maximum capacity and feel your chest expanding. When your lungs are full, hold your breath for 10 seconds. Release the breath, and begin a new round. Repeat 3 to 4 times.
Step 5: Enjoy the feeling
Take your time to enjoy the feeling that the breathing exercises invoke. This feeling is similar to the effects of meditation and will get more powerful when you practice more often.