For this exercise, you will need to use a leg extension machine.
First, choose your weight and sit on the machine with your legs under the pad (feet pointed forward) and the hands holding the side bars. Adjust the pad so that it falls on top of your lower leg (just above your feet).
Also, make sure that your legs form a 90-degree angle between the lower and upper leg. If the angle is less than 90-degrees, means the knee is over the toes which in turn creates undue stress at the knee joint. If the machine is designed that way, make sure that when you start executing the exercise you stop going down once you hit the 90-degree angle.
Exhale and use your quadriceps to extend your legs to the maximum. Ensure that the rest of the body remains stationary on the seat. Pause a second on the contracted position.
Inhale and slowly lower the weight back to the original position, ensuring that you do not go past the 90-degree angle limit. Repeat.