Painful pose is similiar to low lunge except the front foot is turned out 90 degrees.
If you start in a low lunge (below left), then first turn your foot out and then move your back knee further back so that your shoulder is over your foot (below right).
If your left foot is forwards then try to position your body so that your left shoulder is over your left foot.
Lift the inside edge of your front foot as you push your knee back. Imagine trying to lay the side of your thigh on the floor.
You may find the pose is less painful if you shift your your weight slightly towards your front foot side and then press your knee back. The idea of this weight shift is to add tension to the outside of your front leg.
Keep your elbows straight and focus on pushing your front leg knee back and down. To add weight to the pose, you can lift your back knee.
To deepen the glute stretch place your elbows on the floor. First place your outside elbow on the floor. Then sink your other elbow to the floor.
To add weight to the pose you can again lift your knee. Or if you like, inhale and lift your knee and then exhale and lower it.
Finally, as shown below, you can come back onto your hands and then bend your elbows to sink your chest to the floor. Try to get your shoulder to your foot or ankle. Press the knee back at the same time. And if you like, lift your back knee.
From here you can move on to armpit pose.