Find a box or bench that is roughly knee height or a few inches lower. (The lower the box is the lower the mobility requirement).
You can either place your toes or the top of your foot on the box. Point your front foot forward or slightly turned outwards and break at the knees and hips simultaneously.
Aim to get your front leg to hit parallel depth (hamstring parallel with the ground) or lower.
The further forward your front foot is, the longer the Range of Motion is at your hips. The further back your front foot is, the longer the ROM at your knees.
Sit on the machine with your back against the cushion and adjust the machine you are using so that your knees are at a 90 degree angle at the starting position.
Raise the weight by extending your knees outward, then lower your leg to the starting position. Both movements should be done in a slow, controlled motion.