The first steps to coming up with a plan can be overwhelming. Just remember, start small[1] and build up over time. Just a single paper helping is better than nothing. A single letter of affection could make the difference of a broken heart and rekindled resolve.
If you are lost, start with something you can do in ten minutes (and maybe a quick trip to the grocery store):
Find a large envelope you can seal
Every once in a while, you want to expand on the details beyond just the passwords:
Review what you write on occasion and expand on it
While you are doing this, you want to figure out who can you can trust to do things:
Figure out who you can trust to help
Figure out who can help your trusted folk
Find someone to take care of your children and pets
Select someone to handle financial information
Write letters and thank them, put it in your file
Write up notices and post them various places
There is also a knot of legal tasks you want to get done. Thankfully, these don't require much effort once they are done, but you'll still want to review them occasion.
Find a way of handling incapacitation
Below are various useful links within this site and to related sites (not all have been converted over to Gemini).