Introduction
Every website for each account is totally different regarding where and how you find information. That's why it's so important to document everything about each website for your family since they probably are not familiar with it at all.
You will also find this process useful because you will probably discover new things about the websites as you examine each one for what you need to describe in your notes. Like me, you may find some things that need updating.
So each week I focus on a specific website, and document the account note with my commentary (and screenshots wrapped in callouts) from the perspective of what my family needs to know and do with that account website. Then we review that account note and walkthrough the website at a Family Review meeting.
Unique websites
There are many accounts and each website is unique. Although they all have some common elements such as structure, content, and support, each one presents that information in a different way with various ways to find what you need.
For each account, you need to know much more than just the login address and credentials. How do you find what you need to know and do once you access the website? I map each account website to the common structure of my account notes.

Account notes
I found it really helpful to document everything I know about an account so these become an account user guide with all the navigation guidance, procedures, and screenshots so my wife can handle everything without me. It will probably also be necessary to cover some technology skills along the way as well.
Account notes are even more important today since most accounts are probably paperless with everything delivered online rather than in postal mail. This requires that you document workflow procedures for each account that include receiving email notices, accessing websites, downloading documents, and filing in document archives, which I will cover next.