All of these hip extensor stretches are done with the knee of the target leg bent. Bending the knee takes the hamstrings out of the stretch so that you can focus on the single joint hip extensors.
Low lunge yoga pose is a great way to stretch the muscles at the back of the hip, the hip extensors.
As a warm up you could use an unbound marichyasana variation.
Another stretch you can use to stretch the back of the hip is happy baby pose.
One of the easiest hip stretches for the back of the hip is to kneel on one leg with the other foot flat on the floor, knee upwards.
This is a marichyasana yoga pose variation.
You can sit on the heel of the kneeling leg or have the hip inside the foot (like in hero pose.) Place both hands on the floor in front of you and bend forwards. If you find your lifted knee uncomfortable, try moving the foot forwards. I'd suggest positioning the foot so that the front of the ankle is just behind the front of the knee. Rather than letting the knee move outwards use the inner thigh muscles to pull the knee inwards, towards the chest.
From here it's relatively easy to reach the kneeling leg back so that you are in a low lunge.
With one leg forward, and the other back, focus on sinking the chest down towards the floor.
You can start with the back knee on the floor.
But to add weight to the hip extensor stretch, lift the back knee.
In the picture to the right I'm doing low lunge with the top of my back foot on the floor.
You can also do it with the toes of the back foot tucked forwards.
This pose has a similiar effect to a hamstring stretch except that it stretches the single joint muscles that open the front of the hip.
To stretch the hamstrings, the front knee in this pose would have to be straight.
If you find hamstring stretches difficult, you may find it helpful to start with low lunge. You'll then be reasonably sure that the restriction in your hamstring stretches is actually the hamstrings and not the single joint hip extensors muscles.
Generally in low lunge, position the front foot far enough forwards (or back) so that the front of the knee is just in front of the front of the ankle.
A general assumption is that having the front knee too far ahead of the foot is bad for the knee. That may or may not be true.
If you do find that you get knee pain try adjusting the position of the front foot. Additionally, try to activate the front foot. You can also try activating the muscles of the inner and outer thigh and hip.
You can use three arm positions when doing low lunge to gradually deepen the hip extensor stretch.
For the last variation, the lower you can sink your chest, the more you'll stretch the hip extensors of your front leg. You'll also be strengthening your arms at the same time.
Lifting the back knee in low lunge adds weight to the stretch, helping your pelvis to sink down.
To lift the knee focus on pushing the back foot down, into the floor.
You can have the foot pointing back (and thus stretch the front of the ankle) or tuck the toes forwards.
Low lunge with back knee on the floor and with knee lifted.
For each arm position you could start with the knee down for a few breaths and then finish with the knee lifted.
Happy baby pose, whether upright or reclining, is another option or substitute for low lunge. It too can be used to stretch the hip extensors.
Both poses require arm strength but the seated variation may force you to use your abs more. That being said you might find it helpful to use your abs in the lying version of happy baby hip extensor stretch also.
One final hip extensor stretch is to do low lunge with the back shin against the wall. This then stretches the hip flexors as well as the hip extensors and can be a good preparation for the splits.
You can start with your forearms on the floor (or on your hands with elbows straight).
If you have your hands in a push up position, you can sink your chest and then increase the hip extensor stretch.