These stretches are to be used at the end of work-outs to avoid (too much) muscle soreness but can, and should, also be used regularly on their own purely to maintain and increase suppleness and flexibility. Stretches should always be performed when the muscles are warm
Never stretch into pain, your body will react against the pain and prevent the muscle from releasing and stretching.
Try and feel the stretch as soon as it starts to happen, then hold it for 10-15 seconds and when you feel the muscle relaxing and releasing increase the stretch a small amount and hold for a further 10-15 seconds. This can be repeated several times during each stretch.
Your muscles and joints are able to become more flexible and the stretches will increase over time. Don't aim for too much too quickly, it is counterproductive