Seated hamstring stretch

Sit with legs extended and grab hold of your lower leg at a point where it is a comfortable.
 
Bend your legs and grab hold of your lower leg a an inch (2cm) or so further down.

Straighten your legs and feel the stretch. If it's too painful, you've gone down too far.

Hold for 5 deep breaths. Repeat 4 times, going a little lower each time.
 
Start with a modest stretch and over the days, weeks and months grab hold lower down.

The aim is to be able to grab hold of your feet with your legs straight.

Standing hamstring stretch

You can also do the same exercise standing up in the shower each morning. Make a habit of it.

When you can comfortably get your knuckles on the floor you know you've reached you goal.