Back your feet up against a wall, a box, or against the upper cushion of your couch. (If using a hard floor, put down a cushion for your knee).
Slide your left leg so that the knee fits into the corner where the floor meets the wall (or box or cushion). Try and get the shin completely flat on the wall and point the toe.
Squeeze your glutes especially your left glute. Keep squeezing throughout the stretch. This will help to stabilise your lower back and correctly position your hip joint.
Bring up the right leg and place the foot flat on the floor in front of you with a vertical shin.
Continue to squeeze your glutes and maintain this position for at least a minute.
Then if you are able to increase the tension, lift up your torso (glutes still engaged) and hold for another minute.
If you're too tight to get into the Couch Stretch position, scale things back by positioning a box out on front of you that you can put your weight on and not worry about bringing the leg up.
Work on this daily for at least 2 minutes each side.