Before I get to the breathing, a little background on Wim Hof...
Wim Hof is a holder of many world records for things like spending a long time neck deep in ice, swimming under a frozen lake for long distances, climbing Mount Everest without a shirt on, and running miles in the snow barefoot. He's been called the Iceman for good reason. These are just a few of the amazing things he's shown humans can do. And that's his whole point, anyone can master their body the way he has. Through breathing and mindful awareness, one can improve the overall physiology of the body. This breathing exercise is meant to saturate the body with oxygen and help balance its ph.
Take the deepest breath you can, all the way in until you can't inhale anymore, then release in such a way where you don't force or blow out the air, but simply relax and let it go. After no more than a second or two, inhale again. You're going to keep taking super deep breaths a few dozen times at least. Remember to treat each inhale like you just came up for air from under the water, force as much air into your lungs as you can, then relax on the exhale.
After a little bit, you'll notice yourself start to get light headed and tingly all over your body. This is normal and a sign that you're breathing deep enough and doing it right. What you're feeling is your body fill with oxygen, that's why you'll feel lightheaded but not dizzy, you won't pass out. When you feel both light headed AND tingly, all the way down to your fingers and toes, then keep going and do another dozen or so breaths.
At this point you'll feel really light headed and tingly all over. Now you're going to let out one final exhale and, with your lungs empty, just stay still. Don't breathe, don't do anything, just relax and stay still. You'll still feel light headed and tingly, but you'll notice that you have no urge to breathe. You don't need to breathe yet because you have more than enough oxygen already. You won't feel the urge to breathe for maybe 30 seconds, maybe a minute or two, maybe more. It depends on you, how much oxygen you absorbed.
Enjoy.