This video shows a simple hip flexion exercise which is also a hamstrings stretch.



In case you cannot see the video, here is a still sequence -

hip flexions

Hip flexions include both an active component and a stretch. Sit on a firm table-height surface with one leg outstretched and the other resting on the floor. 

hip flexions

Bend forward from the hip in line with the extended leg. Hold that position for a few seconds.

hip flexions

Sit upright again and repeat this 5 times.

Change position and repeat the sequence with the other leg.