The leg extension machine focuses mainly on your quads (quadriceps) muscles as well as the rectus femoris and vastus muscles..
First of all, adjust the back pad so that your knees are in line with the edge of the seat. You should also adjust the leg pad so that it lines up with your ankle. Once ready, adjust the weight accordingly, hold onto the bars either side making sure your back is in a neutral position against the back rest. Lift your legs upwards, ideally holding at the top, and returning down in a controlled movement.