This should never be practiced near water, or while driving or walking, or in any other circumstances where you might get hurt should you pass out. Consult your doctor if you are pregnant or have a heart condition.
Tumo Breathing.Find a quiet place and lie flat on your back with a pillow under the head. Relax the shoulders, chest, legs. Take 30 very deep, very fast breaths into the pit of the stomach and let it back out. If possible, breathe through the nose; if the nose feels obstructed, try pursed lips. The movement of each inhalation should look like a wave, filling up in the stomach and softly moving up through the lungs. Exhales follow the same movement, first emptying the stomach then the chest as air pours through the nose or pursed lips of the mouth. At the end of 30 breaths, exhale to the “natural conclusion,” leaving about a quarter of the air in the lungs. Hold that breath for as long as possible. Once you’ve reached your absolute breathhold limit, take one huge inhale and hold it another 15 seconds. Very gently, move that fresh breath around the chest and to the shoulders, then exhale and start the heavy breathing again. Repeat the entire pattern at least three times.
Here’s a link to WimHofs guided session
James Nestor. Breath.