Always set up under the bar in the exact position you plan to squat; feet hip-width apart, hands outside of shoulders, elbows braced back, lats engaged, glutes squeezed and under you, with a neutral spine and pelvis. Do not load a heavy weight onto your frame if it is not set up properly, even if you're "just getting set up". Always set up under the bar in the exact position you plan to squat.
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.
Get air into your midsection by focusing on breathing a big breath into your belly (diaphragm) and low back. This will create 360 degrees of tightness from the pressure, improving performance, stability, and will help your back stay healthy. Get air before you bring the bar out of the rack, breathe out, and take another big breath before you do each heavy rep, not during. Keep the air tight in your belly and back until you hit the bottom of your squat, and breath out through your belly (diaphragm) as you squat out of the hole, to your top position.