This is a vital document. Not only does the letter of last instruction spell out your wishes in many areas, it also lists all manner of important information that whoever is settling your affairs needs to know. Things like bank account numbers, insurance policies, mortgage information, car titles, even lists of people who should be notified. (In all likelihood, there are a lot of casual friends and acquaintances who the people taking care of your affairs don’t know.)
In addition, buy a small ‘internet address book.’ Record the access information for all online accounts, memberships, and emails. That includes usernames, passwords, account numbers, and answers to security questions.
Both of these need to be updated every year and kept in your safe deposit box. A second copy should be with documents at your house.
The University of Idaho Extension Service has a very detailed form to write a letter of last instruction. It walks you through each of the categories and how quickly items need to be addressed: Who to notify and when, bank accounts, insurance, safe deposit box (where is the key?), cars, where to find personal papers like diplomas and baptismal certificates, household items you wish to give away and to whom, even funeral details. If it is not covered by this template, it probably doesn’t need to be included. You can download the pdf here: http://www.extension.uidaho.edu/detail.aspx?IDnum=2636
BoomerBaggage.com, a site with a lot of information about financial planning for Boomers, has a much less detailed template that is a good starting point. (https://boomerbaggage.com/letter-final-instructions/