Get under the bar.

With the bar racked around nipple-height, center yourself on the bar and plant your feet directly under it around shoulder width. With a short, sharp exhale, squeeze your glutes and tuck your ribs a bit.
Set your feet under the bar, at about shoulder-width apart. I’ve seen countless squatters in reader submitted videos who take the bar out in a split stance. This may work on lighter weights, but it will hold you back at maximum attempts because it can throw you off-balance.
A lot of people are going to stand back [away from the rack] and you know this is wrong because when they stand up, the bar comes [back instead of up] out of the rack... This will work at 50%-60% but once you get above 80%, it's gonna make the bar feel really heavy because you have to lever up with your back to get it out. Instead we need our feet to be forward and under the bar.
John Paul Cauchi
As you’re squeezing under and getting tight, think about setting your feet directly underneath your hips and parallel to each other. DO NOT SET YOUR FEET IN AN OFFSET FASHION!
Robertson Training Systems
Start with your feet a little bit closer than you would normally because you're gonna have an easier time...
Exhale Ribs Down
Before you approach the bar, exhale hard to allow the ribs to come down and the pelvis to come up and underneath you slightly... Get the air out to optimize your starting position, and then approach the bar.
Robertson Training Systems
Before you start, take a forceful exhalation... draw your ribs down. Keep your abs, then breathe in through your nose.
Dr. Jordan Shallow