I use a cinch from a saddle that will fit around the donkey's neck. I tie the calf to the two rings on each endof the cinch, and tie it at approx 1 foot in length, so that the calf is always beside the donkey. I also agree that the quieter, and gentler the donkey the better it is at halter breaking. We have three, and the best one is the timidest one of them.
Last year, I had a bred heifer that I tried to tie up for the first time in the barn. She was jumping, flipping herself and smashing her head so much that she had blood coming out her nose. I was afraid she was really going to do some major damage to herself, and I had to leave for a few hours, so I did not think I should leave her tied in the barn. I tied her to Elizabeth and turned them into a small pen. I cam back three hours later and they were standing in the same place I had left them. I untied the heifer and led her back into the barn and she never even pulled on the halter. I am not sure what magic Elizabeth put on the heifer but she was halter broke from that point on.
Elizabeth is a sweet heart but I still think she looks more like Charles. She is pictured on our website.