Be in a strong athletic position: knees flexed, core and glutes activated, chest up, and shoulders back.
Get underneath the bar, placing it on your traps.
Have your hands outside shoulder-width in a position where you can create torque in your shoulders. You want to keep your shoulders pinned back. The equates to a strong upper back position.
The bar does not sit loosely on your back.
Imagine pulling your elbows under the bar with your chest up and shoulders back.
You want your torso in a strong, vertical position.
Chest up and eyes fixed straight ahead.
Unrack the bar and step back one foot at a time.
Place your feet just outside shoulder-width with your toes pointed out slightly.
Stance will vary depending on the person.
To create stability in your hips, screw your feet into the floor and push your knees out.
Push your hips back slightly and descend into the bottom position.
When the crease of your hips reach level with your knee, accelerate back up.
Continue to push your knees out and maintain your core tightness.
Squeeze your glutes at the top position.
*Tips*
The barbell back squat takes time and many reps to solidify technique.
Practice, practice, practice before you load up the barbell with heavy weight.
Always look straight ahead, never look directly up or down. You want to maintain a neutral neck position.
Don't overextend/arch your back. Focus on core tightness and a neutral spine.
In the bottom position, imagine driving your chest up into the bar.