Stand in an upright vertical posture next to a stationary post, power rack or chair, etc. With a slight absence of knee lock, place your feet approximately 12 to 18 inches apart, with heels inward and toes rotated out laterally, just slightly. Vince Gironda says the feet should be 13" apart and the knees 17" wide.
To maintain a perfect balance in this "fire-bombing" quadriceps exercise, lightly grasp hold of the stationary post, etc. with one hand.
Now, with just your own bodyweight, rise up on your toes or, if you wish, place your heels on a 4" x 4" block of wood. Lean your upper torso backward (approximately 45 degrees from vertical) until you feel a maximum stretch contraction in the quads, especially just above the knees. Your upper torso and thighs will be in alignment with one another if you have done this correctly.
While maintaining this inclined, lying back position (you will basically be at a 45-degree angle to horizontal position), slowly lower your body by bending your knees, allowing them to thrust forward. Allow the upper torso and thighs to descend to where the shoulders are directly over the heels and beyond. Do not relax at this point. Keep continuous tension on the quads by doing a smooth direction reversal at the bottom of the negative stretch (approximately parallel to the floor) phase by straightening your quads and driving your hips forward till you are once again at the non-lock starting point. Remember, as you come up, to push off on your heels while pulling the front part of your foot up off the floor.
Begin the next rep immediately. With absolutely no pausing, continue until you have completed 12 to 15 maxi-pump reps in nonstop, nonlock style.
It is a very good idea to practice the sissy squats with just your own bodyweight until it becomes a natural movement. There is a saying, "Practice makes perfect." I prefer to take it one step further and say, "Perfect practice makes perfect." This makes sense because, if you practice the Sissy squat or any other exercise for that matter, using sloppy form, you will never develop a precision technique.
Once you have mastered a precision technique with your own bodyweight you can begin to use extra weight in the form of a cast iron plate or a dumbbell or dumbbells. Of the three options, the loose plate is the easiest to accommodate because all you have to do is hold it securely against your chest with your free hand while maintaining perfect balance with the other. Holding a barbell (as in a Front squat position) or holding dumbbells hanging at your sides does not allow for the degree of leaning back you achieved with your bodyweight alone. The reason is that your balance is compromised because your hands are not free to assist you. This is a very minor obstacle to overcome. You can attach a 4-6 foot length of ½" rope securely around your waist (or tie it to the front of your lifting belt, directly in the center) then tie the other end of the rope at chest level to a stationary post. This will free up your hands so that you can use the barbell or dumbbells, and at the same time allow you to maintain the proper inclined layback position - and with perfect balance.