Breathe in.

Recover any breath (and tension) you lost during the walk out with a quick inhale. This position is the only place you’ll inhale or exhale throughout a set - whether it’s 1 rep or 20.
Recover lost breath
The most advantageous position for you to actually pull air is before you have the weight on your back... You're getting everything tight, then pull as much air as you possibly can and then when you stand up with it, you can let a little out and then you know pull that back in again.
GO UNDERWATER
GO UNDERWATER
whiteboard_daily
I'll take a big [breath] at the beginning through my nose and then take [the bar] out. I don't want to let it all out because... you're actually loosening up... You want to keep it locked in. So as you walk back you're letting out a little bit of air and then getting more in. Just enough to do the exercise itself and you don't want to take in too much of it through the mouth because you're not filling up your lungs well.
Super Training
Inhale before you unrack the bar and walk it back. Inhale again when you’re ready to Squat, right before you go down.
StrongLifts
There is breathing time and there is lifting time and the two are not friends... You should get a big belly breath and you should brace it down, but from the moment that that bar begins descending you should not be [exhaling] whatsoever until you are 90% done with that squat. Filling your torso with air and holding your breath that's something that your body does naturally to help protect its spine when you go to pick up something heavy.
Fundamentally, you want to be breathing in only at the top before you descend, and you want to breathe out towards the last half of the movement or even just breathe out at the very top. You don't want to be breathing in as you go down and out from the very bottom.
Renaissance Periodization
So once I've gotten my air in, I brace my abdominal wall. I'm going to hold that throughout the duration of the movement until I finish the rep and then I'll blow it out forcefully. If I'm doing multiple reps I'll then take a new breath in at the top of the rep and then descend, do the next rep, and repeat.
Super Training