1. Install Matrix
2. Install Element
3. Using Element, create an account using your Cloudron credentials
4. Make an existing user an administrator...

This 4th step is the tricky one. The documentation explains how to do it:
Open a terminal and run the following command:

PGPASSWORD=${CLOUDRON_POSTGRESQL_PASSWORD} psql -h ${CLOUDRON_POSTGRESQL_HOST} -p ${CLOUDRON_POSTGRESQL_PORT} -U ${CLOUDRON_POSTGRESQL_USERNAME} -d ${CLOUDRON_POSTGRESQL_DATABASE} -c "UPDATE users SET admin=1 WHERE name='@user:example.com'"

However, some of this command needs to be edited by the user, I think, and this editing should be mentioned in the documentation.

If you do edit the command to use your own username, for example:

@LoudLemur:exemplary.com

and then run it

PGPASSWORD=${CLOUDRON_POSTGRESQL_PASSWORD} psql -h ${CLOUDRON_POSTGRESQL_HOST} -p ${CLOUDRON_POSTGRESQL_PORT} -U ${CLOUDRON_POSTGRESQL_USERNAME} -d ${CLOUDRON_POSTGRESQL_DATABASE} -c "UPDATE users SET admin=1 WHERE name='@LoudLemur:exemplary.com'"

You would expect to be able to visit the matrix/synapse page and find a login panel. There isn't one to be seen. Perhaps you are just meant to login to Element as the user you just made an admin, but where do you find the Synapse section?