From https://support.opensciencegrid.org/a/tickets/10135, we don't have information on how to use Let's Encrypt certs on EL6 because certbot is not available in standard repos.
Our instructions do work with the certbot-auto script mentioned at https://certbot.eff.org/docs/install.html#alternate-installation-methods so we should mention that.
Because of the nature of the script (you download it from the web and it can update itself), we should add the following advice:
Use the instructions on the site to verify the script after downloading it
Modify the cron job to add --no-self-upgrade to the command line.