These are few simple stretches for people with weak lower back and hamstring in exercises. Each stretch should hold for 8 to 20 secs, according to the person's physical ability. Stretching would improve one's flexibility and indirectly assists in daily functional movement. It would improve reaction and reflectional action if necessary.
You could get detailed instruction from any certified gym trainer if you need correct practice procedure. Please note that the photographs here are only for rough ideas of these stretches, they do not reflect the actual exercise protocol.