Quads/ leg extension

Make sure the seat is in a good position so that your knees are actually bending over the front, rather than trying to bend further back!  Hold the hand grips and straighten the legs slowly, pulling up the toes momentarily when the knees are straight (you'll feel the calf muscles tighten).  Lift at a steady rate and don't snap the knees back - people tend to do this if the weights are too heavy.  Make sure that all the effort is in your legs - you will see people trying to lift too much and they'll arch their back and use other muscle groups.  When you increase the weights, do half sets (5 reps) until you get used to it.  If you get to a stage where one weight is easy and the next seems too hard, put one of the hand weights on the stack to give you an intermediate value.

Hamstrings

knee flexion

Lie face down with your hips over the point of the bench and heels under the roller.  Make sure it’s on its shortest setting (no holes showing) or you'll be trying to lift with the wrong part of your leg.  Bend you knees and pull the roller up to your bottom.  Lower slowly until there's a 1cm/finger width gap above the blocks and then lift again.