Smith machine squat:
Adjust the bar of the Smith Machine so that it's at shoulder height. Ultimately, the height will be different for everyone, and you may need to raise or lower it. Position your hands about shoulder-width apart on the bar, and lightly grip the bar with an overhand grip. Step in front of the rack, and rest the bar on your trapezius muscles (the muscles closest to your neck/upper back). With your feet about hip-distance apart, unlock the bar by lifting it up and forward. Shift your weight back into your heels. Brace your abs as you begin to lower into a squat, keeping your head and spine in a neutral position. Your knees should be as close to 90 degrees as possible. Hold for one second. With your core still braced, drive through your heels to stand back up. Be sure to squeeze your glutes at the top of your squat.
Mistakes and Recommendations:
Do not be afraid to take your pelvis back enough. In other case you can injure your back.
Smith machine squats will not allow you to fall, do not be afraid to stand in the right position.
Do not let your knees go too far beyond your socks. It ruins the technique!

Smith machine squat.