Rack your back.

Duck your head in front of the bar and grip it as narrowly as you can with straight wrists. Pull your elbows down and in toward your ribcage, so that the bar is resting on the flexed muscles of your upper back. With a tight grip, pull the bar down and apart while digging your upper back into it so that it feels like an uncomfortable and immoveable part of your body.
Grip
The closer to center you can keep your hands on the bar, the thicker and more stable shelf you create... Unless your shoulders are too beat up, you’ll be better off inching them in.
The correct grip places the wrists into a neutral position and the forearms and elbows are closely aligned with the torso. The typical errors we see in this step involves trainees rolling their hands up over the barbell, putting their wrists into flexion, and holding the bar in place with the heel of their palms often with their fingers off the bar entirely. This raises the elbows far too high causing... the bar to roll up the neck... The wrist should be put into a neutral position... and if the lifter can't maintain a neutral wrist, then a little bit of extension is preferable to flexion.
Alan Thrall
...a narrower hand position will help you keep your upper back a little tighter and more stable. When you bring your hands in, your scapulae naturally have to retract harder (so your middle and lower traps will be tighter, along with your rhomboids), and your shoulders will naturally have to adduct (tensing your lats a bit)...In general, your hands should be as close as you can comfortably get them. If you can get them closer without pain in your wrists, shoulders, or elbows, or just feeling super uncomfortable, then you’d probably benefit a little from doing so.
Stronger By Science
The closer you can put your hands to each other on the bar the easier it will be to have a tight setup. Make sure though that your elbows are inside your hands to maintain tightness. Using a thumbless grip often improves the comfort of a close hand position.
When you go with a very narrow grip (i.e. shoulder width), and then crank your elbows underneath the bar... Setting up in this way gives you a ton of stability and control through the upper back, and it also helps keep you from getting pitched forward with the bar on your back (which is never a pleasant experience!)
Robertson Training Systems
...pull your grip of the bar in just a little. This will force your shoulder blades together and fire up your upper back and erectors.
In the low bar squat, the bar should be resting on your posterior deltoids. [Any pain] generally comes from letting the bar rest directly on the spine of the scapulae. Just moving the bar a teensy bit higher ... or lower should take care of that discomfort.
Stronger By Science
If I had the bar on my back and my wrists are cocked back, that's not gonna be a good position for my elbows; vice-versa if I have my knuckles pushed too far forward you can already see that it's driving my elbows back. A way to correct this is just to have a neutral position with the wrists... keeping it straight... versus rolled forward or backwards.
Szat Strength
Generally, the narrower your grip the better position you're gonna have on your back... you're gonna have a very nice shelf on your upper back made by your delts - very nice shelf to hold the bar. The ideal grip is gonna be the narrowest you can go without hurting your shoulders.
Once you've got your hands placed where you want them, squeeze the bar as hard as you can. The harder you're squeezing that bar, the more muscles you can recruit.
I want you to grab [the bar with a] full hand... so now my whole hand is on the bar. When I wrap around my pinkies are on there as well. That's going to distribute the force throughout the whole arm and upper back to allow you to keep your upper back tighter. [Otherwise] all that force is creating bicep tendinitis.
Elbows down
KEEP YOUR ELBOWS DOWN
KEEP YOUR ELBOWS DOWN
TOUCH YOUR TRICEPS TO YOUR LATS
TOUCH YOUR TRICEPS TO YOUR LATS
Juggernaut
Another novice mistake is to point the elbows backwards during the squat. Not only is this bad for the shoulders, but it also creates a poor foundation for the bar to rest on and makes it easier for you to round out in the bottom of the squat. If you have difficulty pulling your elbows down, you can grip the bar with your first three fingers only, it takes some getting used to, but a lot of big squatters have improved their technique this way.
...your elbows should be down and pulled into your sides... “scratch your rib cage with your elbows.” This will help create lat tension to aid in torso rigidity and upper back tightness.
Stronger By Science
Squeeze your elbows towards the middle of your body to improve upper back tightness in the setup. Once you squeeze them toward your body as hard as possible, force them forward, under the bar. There will be very little movement of the elbow forward if you are doing a good job of squeezing them in.
I cue my lifters to pull their shoulder blades/scapulae back, and then rest the bar on top of this “muscle shelf.” Even the lightest, boniest human being known to man can do this without resorting to a pad or towel in between themselves on the bar. When you place the bar too high on the back, it ends up resting on the neck. On the other hand placing the bar too low again puts a ton of demand on mobility through the shoulders... you end up arching the upper and lower back in an effort to compensate.
Robertson Training Systems
What we're looking for is for the line of the elbow (in terms of the forearm) to be relatively in line - or parallel - to the torso. Especially as you descend into the bottom of the squat. Now often times when we're at the top of the squat, the line of the forearm is gonna be slightly behind... a good cue that I like to use is to try to get people to pull their elbows together behind their back. Maintain that tightness - that feeling of pulling the shoulder blades down into the back - as you hit the bottom of your squat. That's the #1 where people will lose it.
Calgary Barbell
...make sure your chest is up before you ever unrack the bar! To help in getting your chest up, think about spinning your elbows down and underneath the bar. Again, this will help lock in your upper body/torso position and get you ready to squat!
Robertson Training Systems
So you want to elbows to be directly under the bar the entire lift. Your torso more or less is gonna follow your elbows. If your elbows are pointing down, your torso is gonna be pointing up. That's what you want. So try and keep that position as you descend.
Bend the bar
BEND THE BAR
BEND THE BAR
...you should actively pull your shoulder blades together.
Stronger By Science
...make sure that you are squeezing that bar with everything you have ... I want to [be] taking that bar and pulling it through your back like you're trying to rip it out the front of your chest ... because the more that you can meld that bar to your body ... visualize trying to bend that bar across my back like it's a horseshoe ... I'm trying to rip the bar through my body and come out a couple inches above my nipple line.
Neversate
I think the easiest cue for a lot of people is to think about pulling the bar apart or squeezing the bar over your back... Personally that doesn't actually work for me. I like to shrug up and then pull down. So [I'm] shrugging up and going to tighten up everything that's in this upper back and then when I pull down I'm gonna kind of let my traps relax. I don't want tight traps; that's gonna prevent me from using those muscles effectively.
Squeeze everything towards the middle of the body: shoulder blades fold together, elbows pull together slightly behind the body... So basically your elbows are gonna end up pointed at the top of your butt, and finally... push your elbows under the bar.
Juggernaut