We did 1 cow in an arena, and then began pushing the cow away from us at a walk. Once the horse learns that he is the boss and can push the cow, he gets a lot of confidence. Then you start teaching the horse how to manuver the cow, and if the horse is confidence and focused on his job, he will learn how to use his own body language to help you. Then add more cows.
I've had worse trouble sacking out horses to the electronic cutting cow than the real ones so... it's not so bad if you have a controllable, cowy horse. I personally don't like the horse to get too "hot" around the cow. The very beginning would be the time to nip that in the bud. Push the cow, rollback away from the cow, push the cow again, figure eight behind the cow, etc.