How do you keep from tearing from anal sex?

Lots of lube. Use lots of lube. Be gentle. Go slow. If it hurts stop. It;s ok if you can't have anal sex. Some people can't do it.
Lubricate & Patience. Use plenty of water soluble lubricant and have partner insert very slowly, stopping if and when there is any pain. Anal recipient must try and bear down softly and above all relax.