Before going underwater, spend five minutes breathing in and out slowly like you've practiced. Relax your body as well while sitting or standing in the shallow end of a pool or body of water. Take one deep breath through your mouth and lower yourself below the surface of the water. Keep your mouth and nose closed while underwater.  Use your fingers to hold your nose shut if you need. Staying relaxed is important since there is more risk in holding your breath underwater than on land. Once you've reached your body's limit, swim or push yourself to the surface. Blow out any extra air as you rise so that you can take a fresh breath immediately.  Before diving again, take another two to five minutes to do a few breath cycles to get your body back to normal oxygen levels. If you begin to panic at any moment, relax and surface. Panicking can lead to accidental inhalation underwater, which can cause drowning. Swimming and forcing your body to dive deeper will use up more oxygen. Don't try to exert yourself right away.  When diving, you want to remain as relaxed and calm as possible, keeping your pulse low. Swimming is the complete opposite. Your pulse will be high and your muscles moving quickly. As you begin to go longer without breath, avoid using a timer or counting seconds because you will mentally psych yourself out. Instead, measure how far you can swim across the pool or how deep you can go before needing air. If you want to keep track of your time, have a friend time you instead.

TL;DR in Chinese:
先以正确的方式练习几次呼吸。 缓慢下水。 缓慢出水。 要是你觉得舒服，可以适当增加身体活动。 不要用时间来衡量自己的进展，要用距离衡量。