Sit comfortably in a leg extension machine. Bring legs up, exhale, put them down, inhale.
Before you begin the exercise you should ensure that the seat and the leg pad are correctly adjusted. First you should sit down on the machine. You should position yourself on the seat so that your knees are at the same place as the pivot point of the machine. You want your legs to turn with the machine. Generally your knees will be slightly off the pad to allow full range of motion. Then bring the backrest up until your back is positioned perfectly flat on it. Finally you want to adjust the leg pad so that it's directly over (just above) your ankle. Now that the seat is adjusted, grip the handles on the seat to keep your body perfectly stationary. Your chest should be out and your back straight. Bring the weight up by extending your legs. Do not lock out your knees. You do not want to turn your knees in or out. This could lead to injury and shift focus from quads core to one to the muscle fibers on one side or another. Exhale, then bring the weight all the way down, but not far enough for the weight to rest on the rack. Inhale on the way down. Be sure to face forward the whole time.
As is the case with most isolation exercises, many people rely on the leg extension to balance leg strength. To avoid over-exercising one leg and not the other, you could perform the exercise with one leg at a time.