On his web site, Jonathan Pond has a Letter of Instructions that you can download for free:
http://jonathanpond.com/letterofinstructions.html
It's an awesome resource. It will include all of the information you mentioned and then some. You could talk your grandma through it interview-style rather than asking her to fill it out if you think that would be easier for her.
It might help if your mom fills one out too and gives it to you (or to another trusted family member) so your grandma knows that you're asking her these questions because they are important and not just because she's old!
My sister & I have exchanged these with each other; I'm 42 and my sister's still in her 30's. It's never too soon.