That is the actual legal term for it. It is a letter for whomever ends up settling your affairs when your gone. Even if you have no will, and the odds are good that if you own no real estate and have no large bank accounts or dependents you probably don’t have a will….yet. Your choice. But EVERYONE should have a Letter of Instruction kicking around, where it can be found. It’s not a legal document so you can say whatever you want.
Things that should be included in this letter: your email(s), provider(s) and password(s), a list of websites where you conduct business or have loose ends to tie up. Do you want to leave your pictures up on Flickr in perpetuity? Do you have things for sale on Ebay, Etsy or Amazon? Do you want THEM to keep your money? Your Facebook, Twitter, Instagram profiles and so forth… as long as your executor has access to your main email they can probably request passwords from all these sites, but why are you making it difficult?
Personally I have all those emails and websites and passwords in one file using Azzcardfile, a tiny shareware program that I run on all my computers keeping the .azz file on Dropbox so I only have to keep the one file updated. In my Letter of Instruction are just directions to call up my AzzCardfile file. You could use any program like Evernote or Google Docs, but my Evernote is like huge.
Other things: location of your tax documents, lists of assets; investments; monies owed you; debts you owe and to whom; contact info for any lawyers or accountants you use; your social security number; preferred charities for donations in your memory.
Do you have pets? List the pets, their foods, their peccadilloes, their vet’s name, the location of their medical records and who has agreed to take your pet once you are no more.
Your home’s information: when is the mortgage due and to whom? house insurance? you don’t want it to get foreclosed while folks are dicking around because you didn’t write a will.
This is also where you would list all that crap in your house that you want to pass on to other people. Does your best friend admire your Patagonia Parka? you better itemize it before your clothes ends up in trashbags at Goodwill. Most of your music is worthless except an original vinyl pressing of a Stones album? list it, so they can put it Ebay, before the rest ends up on Craigslist. Do your parents understand how MUCH you spent on that bicycle? Maybe they shouldn’t yard sale it.
The Letter of Instruction is something you should update annually, possibly with one fist clenched tightly around a glass of white wine.