Holding a plate across your chest with one arm, stand with your feet shoulder-width apart, flat, and pointing slightly out to the sides.
With your free hand, grasp something secure at waist height, such as a fixed bar or bench.
Keeping your back and hips straight, inhale as you flex your knees forward in the direction that your feet are pointing, and allow your body to tilt backward and your heels to rise off the floor.
Descend until your knees are almost fully flexed, making sure not to fall backward.
Exhale as you reverse the motion and return to the starting position.
Repeat for the prescribed number of repetitions.
Comments and tips
Keep your feet and knees pointing in the same direction.
Keep your back and hips straight.
For each set, alternate the hand with which you grasp the bench/fixed bar.
If new to the sissy squat, start with just your body weight (see first video). Use a weight plate for additional resistance only after you have mastered your balance.
Once you master the exercise with the plate on your chest, you can graduate to holding a plate in each hand (see second video).
Do not use the weighted sissy squat as your primary quad exercise, which should be the barbell or dumbbell squat. Use it only to warm up or pre-exhaust your quadriceps before a leg workout, or to burn out your quadriceps at the end of a leg workout.
When performing the weighted sissy squat, some people prefer to keep their heels off the floor throughout the exercise. Try both variations to see which one you prefer.