Set the required resistance taking account of your fitness level. Sit on the seat and anchor your feet firmly against the platform at shoulder width. Legs are bent at your knee joints at an angle slightly less than 90 degrees. Hold the grips on the side of the machine with your hands.
From the starting position when your knees are bent, lead the motion by pushing the platform away from your body. Your legs are stretched fully out in the extended position. Then return to the starting position but do not relax your muscle tension completely. Do the required number of repetitions.
Make sure your knees and feet are in the correct position. Knees must not cross the horizontal plane of your tiptoes. Also make sure to keep your knees in the unchanged trajectory of the motion and do not swing them sideways. Keep your torso fixed and do not bend at the waist, especially when overcoming heavy resistance.
Start with lighter resistance to master the technique well.
Leg press is considered a safe alternative to barbell squat. It is for this reason that many trainees use excessive weights on leg press that can cause the very same knee injury as barbell squats. Remember, there is little difference between leg press and squat concerning the knee joints.
The lower back is spared but also not 100%. It is not uncommon to feel lower back pain after heavy leg presses. This is mostly because of the position of your back during the exercise.
Therefore, caution is necessary. Increase the weight gradually and use knee protection for weights above the 80% of your 1RM.