Definitive instructions for Buster LTS security updates

[Keith Christian <keith1christian@gmail.com>]

I plan to bring a Buster machine which has been shut down for quite a
while online again.
Before connecting it to the internet, I looked for instructions on how
to add the LTS security updates entries to sources.list.

My first search brought me to wiki.debian.org, where I landed on the
/LTS/Using page, but it contained no Buster-specific instructions.

After more searches, I saw numerous posts containing messages like
these, and I decided to double check the procedure to obtain LTS
updates.

E: The repository 'http://deb.debian.org/debian buster/updates
Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is
therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user
configuration details.

Seeing the above, I decided to ask here for the latest info.

Thanks!