What's a good way to prepare for compass yoga pose? Since the leg is behind the shoulder in this pose you can use yoga poses where the shoulder and leg are in a similiar relationship as preparations.
Since this can be harder with the knee straight, start with postures where the knee is bent.
Compass yoga pose can be thought of as a seated hamstring stretch. However it is also a shoulder stretch and a stretch for the side of the body.
Notice the way that the knee (of the straight leg) points inwards towards the body as opposed to pointing backwards.
You could use binding side angle pose. Notice how the shoulder is close to the knee.
If you can't bind then you can do regular side angle pose and focus on reaching your elbow to the floor. Then roll the top side of your body backwards. Again notice the relationship between shoulder and knee.
If you can't get your elbow close the floor in side angle, you might want to prepare for side angle with low lunge.
Aim to touch your chest to the floor in this hip extensor stretch.
Again notice the relationship between the shoulder and leg in this lunging yoga pose.
Similar to side angle but with the leg straight is seated wide leg side bend. (not shown.)
You could have the other leg straight, or bend the knee and fold the shin to the inside of the thigh or outside. In the first case the bent leg knee is in the janu sirsana position. In the second case it is in the hero leg position.
This then could be called gate pose.
In any case, you first reach your elbow down to the floor, and then once that touches, then reach your shoulder to the floor.
In all cases notice the direction that your knee is pointing in.
It points towards the body (upwards) not back.
Before you do compass pose you could do this compass pose prep which I use often as a seated hamstring stretch.
You could also do this shoulder stretch. Which part of the shoulder stretch to do you think is relevant to compass pose, the straight arm or the bent arm?
Getting into compass pose relax the shoulder. Imagine reaching both the arm and the leg up and to the side. Once the knee is straight, or as you straighten it, turn the leg inwards so that the knee faces the body. Then once the knee is straight, then use the arm to pull the leg inwards.
Brace the other hand on the floor, and then so that you can get used to doing this pose standing, take the hand off of the floor. But before you do brace the leg with the back of the shoulder.
What do you do after compass pose?
You might find that it feels right to do half lotus forward bend.
Or ardha matsyendrasana.
Or you might like to do an arm balance variation like easy eka pada koundinyasana.
In all of these poses including compass yoga pose, the thigh is close to the shoulder or in front of it. In the poses where the shoulder presses against the leg or vice versa, notice how this changes the overall feeling of your body. When you press one part of your body against another you create differences in the tension network of your body.
The more tuned in to this that you are the better you can utilize this tension to your benefit.
As an example, hopefully you found that relaxing the shoulder of the upper arm made it easier to straighten your leg. And if you tried the arm balance maybe you found that pressing the leg down against the arm made it easier to lift your body.
Tension can be your friend if you learn how to feel it, control it and use it effectively.