Set up the squat rack so that the bar sits at about collar bone height. Approach the bar with your hands up, palms facing forward just outside shoulder-width. You need to create a "shoulder shelf" for the bar to rest on. This is achieved by gripping the bar and bringing your elbows underneath and up as high as possible so that the bar sits on the deltoids and the upper arms are parallel to the floor. This will likely cause the bar to roll back into your fingertips because your wrist flexibility does not allow you to grip it fully. This is fine and is meant to happen.
Keeping your head up and your elbows high, use your thighs to drive up under the bar to un-rack the weight. Carefully shuffle backwards a half step so that the bar will clear the rack on the decline phase of the movement.
Take a second to adjust your feet position - they should be flat on the floor just outside of hip width and turned slightly outward in the same direction as your knee cap is pointing. This is your starting position.
Keeping your core tight and your torso as straight and upright as possible, make the descent by bending at the knees and inhaling. It should feel like you are sitting back into a chair rather than your knees going forward out in front of your feet.
Once you have reached the point where your thighs are parallel with the ground or just below, you can then commence the ascent. It is advised to choose a lesser weight and squat lower so that your form is correct. Exhale and drive up from the thighs and glutes back to the starting position making sure not to let your knees fall inward during the movement.