Adjust the machine so that when you're seated against the padding your knees are aligned with the pivot point of the machine. Position the bottom pad so that when your legs are almost fully extended out, it rests between the lower end of your calves and above your ankles. Keep your feet straight and your legs aligned at approximately the width of your hips throughout.
Start by contracting your hamstrings to curl the pad down and towards the back of your thighs, go as far as comfortable, then slowly extend back to the starting position in a controlled manner. Tense your core and use the hand grips to ensure your upper body remains stable. Repeat for the desired amount of repetitions, then retract the front pad off your legs to exit.
Avoid the use of excessive momentum, swinging the pad without control.