For this exercise there should be a seat that is tilted and an angle and in almost touching the ground and there should be a metal plate where you put your feet on directly above the seat. Sit on the seat rest your feet on the plate, let it your knees come to your belly and then push the plate out with your legs. Repeat and make sure you do it nice and slow.
1st set 30kg 10 reps (beginner)
2nd set 35kg 8reps
3rd set 35kg 8reps
4th set 30 kg 12-15 reps
Move up by a 5kg every week depending on how comfortable you feel.
[image from workoutlabs.com]