Things that can go wrong:

As soon as you ask him to move his haunches away from you he stops his front end and does a turn on the forehand. First of all watch yourself, that you do not slow down or pull on the reins by accident. Then realize that moving forward is more important right now than stepping sideway. Ask him to go forward and try again. It may take several attempts before he makes his first step forward and sideways at the same time. Recognize that moment instantly, cease asking and praise him.
As soon as you ask him to move his haunches away from you he runs into you. Make sure you did preliminary work with him asking him to step away from the touch of the whip on the haunches. If you didn't then you must stop working on leg-yields and return to previous step. Work on that for a few sessions until your horse understands your request. Check that he respects your space and you always correct him when he steps into it without permission.
As soon as you ask him to move his haunches away from you he tries to move away and when you control his head he stops all together. Do not get upset with him. Continue working alternating the whip and the reins. The quicker you are to release and to ask at the right moment the faster he will understand what you want.
He starts going backwards. Make sure you do not place yourself ahead of his head or even his shoulder. Continue tapping him on the haunches as long as he is backing up and instantly stop when he stops. The quicker you are to recognize that moment the faster you will correct the mistake. When he stops backing up give him a moment of rest and ask him to go forward. Repeat the whole thing if necessary.
He walks faster than you Make sure he knows how to stay with you and how to stop and go when you lead him. Read this article if you are not sure your horse leads well. If he still tries to outrun you (which means he doesn't pay attention to you) stop walking forward do not ask him anything and wait until he comes around you and stops by himself. If you have a barrier behind you it will stop him very soon. While he is turning keep yourself in a proper position in relation to him. In another words, turn your body as much as he turns his. So when he stops you end up exactly where you were before from his point of view. Ask him to back up if you have a wall behind you and start over. If there is no barrier ask him to continue leg-yield. Repeat if necessary.
He is all crooked, bending his neck too much. This will probably be the case if you work with the halter or bridle (reins together). That is why I said in the last blog that working with side-reins will speed up the progress. However, I do not worry that he may be crooked for now as long as his haunches are walking a different path than his front end. The initial phase is more a mental training for the horse than proper execution. As soon as my horse settles into the exercise and understands my requests I change the position of the reins. This will allow me to control his outside shoulder and I do not let him be crooked (I will talk about that in the next blog) . If I start it right away too many aids and directions will confuse and upset him. In a few sessions that I will have with him in the initial phase he will not learn a bad habit. However, he will be much calmer later when I correct his shoulders still asking to go forward and sideways.
He gets very worried and upset. As you may noticed from the previous descriptions of mistakes and corrections the initial phase can be quite confusing for a horse. He may get upset, very nervous, fidgety or simply hysterical. You need to treat such horse with a lot of patience and understanding. You need to give him lots of brakes and work for a very short period of time. The quicker you are with your aids, releasing and asking at the right moment the sooner he will settle down. Stay very calm, focused and positive. I have been working horses in hand for 20 years and I did many mistakes. The hardest lesson for me to learn was to stay very calm and positive. I would get upset and then I will ask more from the horse than necessary or I would miss his correct answer.