Make sure the foot plate is in the correct position for you - if it's too high or low ask someone to help you lower it.
Lie back onto the back-rest and put your feet on the plate. Straighten your knees and lift the side bars inwards (this will take off the brake).
Start by just pressing the weight of the bar and gradually add weights as you feel able. Don't feel you have to go straight for 10 or 20kg either - you can start low (and remember you're also lifting the weight of the bar PLUS any weights you put on).
Always add a weight to each side of the bar or you'll get a skewed lift. Push your knees straight but take care not to snap them back, then bend your knees to lower the weight, and push straight again.
When you've finished straighten your legs and push the side handles outwards to put the brake on - the weights won't then come hurtling down as you try to get out!
You can also use this machine in a 'squats' position where you stand up and bend and straighten your knees. Take the footplate out, lift the seat back and put it between the shoulder supports, stand with your feet where the seat back was, straighten your knees and left the handles in to take off the brakes, and bend/straighten the knees.
Aim for 10 presses and lowers. To work harder add weights to the bar at the back - start low and work up, and always use the same weight on each side. When you've finished put the weights back on the stack to leave the machine unweighted.