Armpit pose is both a hip stretch, a twist and a stretch for the shoulders.
To warm up to it you can use low lunge (sink your chest down), painful pose and ardha matsyendrasana.
To get into "Armpit Pose" or "armpit reach" you start with both knees bent and your left ankle on top of your right knee. You reach your left arm past the bottom of your left foot. If you can get your elbow past your foot, bend your elbow and reach your forearm back under your left shin. From there you can grab your left hand or wrist behind your back.
This version of armpit pose can be quite intense. To deepen the stretch and to relax at the same time, you can tip to your left so that your left knee ends up on the floor with your left foot off of the floor.
In this position you can let your left arm hand from your left foot. The right side of your ribccage can sink down, giving your ribcage a twist and stretching the muscles of your left shoulder.
The first part of arm pit pose is to simply sit with your left ankle on top of your right knee. Have your hands on the floor behind you to prop yourself up.
While sitting in this pose try to get the bottom of your foot to line up with the side of your right thigh. Also position your ankle so that the front part of your foot protrudes beyond the front of your knee. This is to make it easier to wrap your arm around your foot without your right knee getting in the way.
In the next step you'll reach your arm forwards. Prior to that you can shift your left foot and right knee to the left a little. Move your ribcage to the right. Try to get your left shoulder behind your right knee.
Next reach your left arm forwards. Reach your arm past the bottom of your left foot. As you do this try to get your elbow past your foot. You can then focus on getting your shoulder towards your foot. Emphasize reaching the left side of your ribcage forwards so that this pose becomes a twist.
For the next part of armpit pose you have to have your elbow past your foot. If you can get that far then roll your arm inwards so that your elbow starts to point up. Bend your elbow and reach your forearm down and back so that it passes under your left shin. From there, reah your right hand behind your back and try to clasp hands close to the left side of your waist.
Holding on in this position can be challenging. To make it easier focus on relaxing your left shoulder.
You can also try deepening the twist my moving the left side of your ribcage forwards and your right side back.
Finally, see if you can gently tip to the side so that your left knee rests on the floor (shown at right).
Try to hang from your foot and let the right side of your ribcage sink down.
If you are in the mood you can say "Whatz Up?"
As a yoga teacher, I'm constantly exploring new exercises, new ways of doing yoga poses.
There is no single "right way" of doing a yoga pose. Instead, there are options. And the better you are at "feeling" your body, the better you can get at choosing the right option for your body as it is now.
For any technique, the point of practice is to learn feel it and to control it, so that it can be used without thinking about how to use it.
And that is more or less the approach taken in all of my ebooks and videos. They help you to feel your body and control it so that you can work towards using it effectively in anything that you do.