Adjust the machine so when you're seated your knees should be aligned with the pivot point of the machine. Position the back pad so that when your legs are almost fully extended out, it rests between the bottom of your calf muscles and above your ankles.
Secure the top pad firmly on the top of your quads but not too far where it's pressing on your knees. Your feet should be roughly straight and have your legs spaced at approximately the width of your hips throughout.
Contract your hamstrings to the curl pad on the way down as far as it feels comfortable, briefly hold at that point then slowly extend back to the starting position in a controlled manner. When conducting the leg curl ensure that there is no upper body movement, to remain stationary its common to use grips on the sides.
Once you have completed the desired amount of repetitions finish with your legs extended out and retract the front pad of your legs to exit the machine.