enable my father to sell co-owned property?

You need to go to a real estate lawyer he will advise you what documentation is needed. Remember it is what they go to school for.

You need to get a durable power of attorney form (from an office supply store) and have your father sign it in the presence of wittnesses. He has to be of sound mind and sign it of his own free will. It doesn't hurt to get it notorized either. It is possible that it may be challenged if he signs it right before the property is sold. These durable power of attorneys are usually made years in advance of illness and debility so that someone can take care of a person when they need it at a later time. I hope you are not taking advantage of your father.