This is a tutorial on shoulder stability in doing the downward dog pose. The self test of flipping your forearm 180° will tell you whether you should be doing downward dog, because as basic as the pose is, you may have to adapt (with Dolphin Supinate pose) to work on external rotation of the shoulder girdle if you can’t get your hands flat as shown in the test.
Also, do make sure you test both arms. It is very common to find ‘work arounds’ in doing stretches. I realized that at least at the moment, my left shoulder is externally rotated but my right shoulder isn’t. My right hand is about a 45 degree angle! That right shoulder needs some help!