Set up the Smith Machine so that the bar sits at about collar bone height. Place the hands either side onto the bar in an overhand grip no wider than halfway between shoulder-width and the plate. A narrower grip with your elbows pointing down will actually help keep your torso upright and make bar balance easier. This will however require good flexibility in the shoulders so start where it is comfortable and gradually narrow the grip as flexibility improves.
Pop your head under the bar and then back up on the other side so that the bar sits on the traps, just below the base of your neck. Using your thighs, drive up under the bar to un-rack the weight. Carefully shuffle your feet forward a half step. This will really increase the focus on your thighs. 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 that 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.