I second Karky. If you watch a freeweight squat you will see the movement will arc. The body does not lower the weight straight down, the torso will lean forward, as the glutes press during the downward movement. A smith machine will lock you body in position.
I use a smith machine for a (creatively named) smith squat. I place my feet much further out, leaning against the bar. This allows me to keep my back completely straight during the movement without placing unnecessary strain on my lower back, hips, or knees. I use this to really focus on quads.
Your lankiness is not the problem, it is your balance. You need to practice. Find a large open wall. Stand facing it, with your toes touching the wall (so you're very close). Know, begin to squat down so your knees are now touching the wall. Keeping your face as close to the wall as possible, attempt to descend into proper squat position. Keep working at this, a little more each day.
This drill will train you to keep your back straight/arched, your knees behind your toes and your head up during a real squat.