The basic side crow possibly, for me, is one of the toughest arm balancers.
The variations that come from it, push up the ante,even more.
Not many people really crack it step-by-step. A few tips to enter this pose neatly:
* Stand up to enter the pose instead of trying it from a seated squat.
* Do side-planks to strengthen each wrist and forearm.
* Be prepared for the surprise shift into one arm when you enter the pose.
* Unless you stress the forearm, through regular practice,even if only for a short time, your arms will never understand the special strength needed for it.
* Never forget the most important thing: maintain a focal point on which to fix your eyes. This will somehow make your mind calm enough to enter it more confidently.