Find a good attorney with estate, power of attorney and living will experience (I know, the easy answer).
He/she should also be current on the language used by various entities like banks & etc - they parse every word.
I went rounds with BofA for example, and one of Mom's investment outfits, even though there was a trust and 2 types of POAs.
Took months to sort out, and that was without vehicles. Their attorney had been a bit casual in some wording.