Lay down on the floor in a door way with your back and hips relaxed and flat
on the floor.

Keep both legs comfortably straight and place your heel up on one side of
the doorway frame allowing the other leg to go through the doorway

Make sure to keep your hips relaxed and flat on the floor, both of your feet
are angled parallel to eachother, and that your not straining you knees.
The stretch should totally focus on you hamstrings. I was very surprised at
how un limber I was as I wasn't able to get my bum very close to the door
frame.

You can use a belt to pull your toes lightly toward your chest to move the
stretch into your calf.

I'm not a PT guy by any means, so this is just my experience, don't pull
nothing. If you have issues, talk to your doc first. I've found it's
allowed me to get better leg extension and feel more comfortable deep in the
cockpit.