Adding to the important information given in the above posts; for newbies, try to clip the head while the clipper head is still cool. Find an area that the animal is comfortable to have clipped and keep expanding that area. You may have to go back to the neck and expand the area forward over the jaw with long strokes. Often the nose is the final challenge, so don't start there. I have never had to leave a head partly clipped while custom fitting.
I believe its a good practice to skin show animals in early spring. This gives them a positive clipping experience on their entire body when they are itching to get rid of old hair. The clippers provide a body massage vs. blocking must feel like a pest summer fly to the animal. BTW, I have one Angus cow that I've spring skinned entirely without halter or chute, just stands beside the cow I was working on. This shows you the memory they have and how they will (or not) trust you.