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The article was written for commands on CentOS but I've adapted them for Ubuntu. I got about a quarter down the page and ran the certbot certonly --manual command and followed the prompts there. For the domain name, I used dns.faithworks.tech.
Is /var/www/html/, which is used in that guide, also the path where the files for your site hosted by lighttpd reside? (Currently just the welcome page it seems.)
Yeah the webroot method is a better solution instead of the manual method if you already have a running webserver on the same host. Not sure why that guide would choose such an elaborate method.
Another advantage of the webroot method is that you can simply run sudo certbot renew to renew the certificate, which usually already runs in the background!
Unfortunately I'm not familiar with lighttpd. Because if the certificate gets renewed, usually the webserver needs a reload or even restart to pick up the renewed certificate. This "reload command" can be added to Certbots --deploy-hook. You can add a deploy hook using the reconfigure subcommand. See reconfigure subcommand for info about the reconfigure subcommand and deploy hook for the --deploy-hook subcommand.