One thing I did that I thought was moderately clever:
I wrote my "Letter of Instruction" which I was fairly certain was complete. But then I realized that often I write stuff and when people read it they either misunderstand what I wrote or I left out some important detail.
So what I did was print out the first several pages of it (everything except my usernames and passwords, basically) and gave it to my sister (who is my executrix) and asked her to read through it and make sure she understood it all and didn't have any questions. Fortunately I had everything in there, but I can imagine she might have said, "OK, but where is the key to the safe?" or "OK, but how do I pay for your funeral?" or whatever. And if she had, I would have been able to fix it while I'm still kicking, rather than leave her to figure it out best she could.
Another thing I'll just add is that it's better to have more than less. My Mom had a "green notebook", and some things were in there like five times. That's OK, we could just chuckle and skip over the four repeats. But one little line in there could have pointed to something that turns out to be important; I think there were a couple of those things.
She also did a thing towards the last year of her life where she got a box (like a banker box lid) and put any important paperwork in there. Then every time I visited, I'd go through those papers and take care of filing them or handling them or whatever needed to be done. I already had a general idea before, but by the time she passed away, I had a really good handle on everything and was able to pretty smoothly and efficiently take care of her estate and then my Dad as he shifted into being a widower.