To use the rear kick machine, simply stand in the middle of the chest cushion and the footplate, and rest your chest upon the cushion and use your arms to grab hold of the cushioned handle in front of the chest cushion. Do these while keeping your legs positioned perpendicular to the ground. After that, simply place one of your feet on the footplate. It is ideal to keep the foot perpendicular to the footplate so as to maximize payout and minimize risk. While keeping your other leg straight, push the raised leg against the footplate, raising the footplate in the process. Make sure that you do this slowly, and do not pop the leg straight, as this can cause injury as you return to your original position. Do this for the number of reps and sets that you feel you can be comfortable with doing.
It is likely a good idea to look into getting themselves a rear kick machine, which is made to be more foolproof than its other counterparts and made to be less difficult to use than the leg press, especially for those who have just started hitting the gym.