Start with two things: the phone passcode and the main email

Why the phone

• Codes arrive there. Banks, email services and shops send a code by text or to an app before letting anyone sign in. No phone, no code.
• It holds the people. The contacts list is how your family finds your friends, your doctor and your accountant.
• It holds the photos, and often the only copies.
• It can lock for good. A phone makes you wait longer after each wrong guess, and some are set to erase themselves after too many.

Why the main email

• Bills and statements land there. A month of that inbox shows what accounts exist.
• Password resets go there. With the email open, most other accounts can be recovered even if a password on your list is out of date.
• Services write there first when a payment fails or an account is about to close.

What to write down

For the phone:

• The passcode. If you also use a fingerprint or your face, the passcode is still what matters, because it is what the phone asks for after a restart.
• The carrier and whose name the plan is in
• The sign-in for the account behind the phone: an Apple account or a Google account

For the email:

• The address and the password
• How the second step works, and where the backup codes are
• Any second email address that receives its reset links

Tell your family to keep the line alive

A phone number that is cancelled stops receiving codes, and the number is eventually given to someone else. Ask your family to keep paying for the plan until every account that texts that number has been dealt with. It is usually the last thing to cancel, not the first.

Use the service's own setting as well

Apple and Google each let you name someone who can ask for your data after you die. It takes a few minutes and works even if a password has changed since you wrote it down. See legacy contacts. Do both: the setting is the route the service honors, and the written passcode is what works on the day.

When you change it, change it here

People change a phone passcode and forget the note they left. Whenever yours changes, update the written copy the same day.