Kneading the hamstrings using your fingers and thumb lying on the floor

Lie on your back with your head on a pillow and hips and knees flexed.
Flex your right hip so your thigh is about vertical.
Reach around the outside and back of your right thigh with your right hand and hook your fingers around your inner hamstring muscles and place the pad of your thumb against your outer hamstring muscles.
Pull on the inner muscles with your fingertips and push on the outer muscles with your thumb while straightening your right knee.

Straightening your leg stretches the hamstrings and adds a counterforce to the pull of your fingers.
To produce greater leverage and pressure on the muscles lift your right elbow and tilt your wrist to the side.
Work along the entire length of the muscle.
Reach around the inside and underside of your right thigh with your left hand and hook your fingers around your outer hamstrings and place the tip of your thumb against your inner hamstrings.
Pull on the outer hamstrings with your fingertips and push on the inner hamstrings with your thumb while straightening your right knee.
Work along the entire length of the muscle.

An option is to use both hands, one on either side of the thigh, and to press your fingertips into the middle of the thigh while simultaneously straightening your right knee – this is particularly effective for treating the short head of biceps femoris.