Breathing is highly important for the powerlifts. It's done the same way for all the three lifts. To breathe correctly, take a breath and ensure that it doesn't all end up in the chest. The easiest way to check this is to ensure that the chest doesn't lift significantly, instead the belly should grow slightly bigger. Keep the air there, now tighten the entire midsection. A valuable cue is to have the lifter push out the muscle at the sides of the belly.
Clear power breathing with a doctor if you have high blood pressure.
This type of breathing and tension have two important purposes: first it's safer for the lower back, second it makes the lifter stronger.
If multiple reps are to be done, a breath is taken before each rep, keeping the air and the tension throughout the entirety of the lift, then taking a new breath before the next repetition.
A more advanced way is to push the air out hard through resistance. It will sound like a hiss. This can be done at the sticking point to create extra tension in the body. Otherwise the air should not be let out until the lift is completed or at the very end of the lift. Slowly releasing the air in a “normal” way of breathing during the lift serves no purpose and comes with the side effect of releasing tension which we of course do not want. The air helps the lifter stay tight and it's tightness that makes the lifter strong.
Many have trouble "breathing with their belly" even though we all did it as children (seriously, look at a small child, they all do it naturally). It's easier to relearn lying down. Have the lifter lie down on a bench and put something light on their belly (a clip or whatever) and tell them to breathe in a way that makes the object move instead of the chest. After they've figured it out, have them repeat it standing up.
This might take some practice and you have to be patient with it. It’s not uncommon to have to remind lifters of this every session and every set for quite some time.