I'm hosting this on Linode running Ubuntu and Nginx and I followed this tutorial at Digital Ocean which was damn simple to go through.
Letsencrypt is making things super easy to get SSL everywhere.
After finishing the setup (15m later) I found that the domain
jj.isgeek.net had the "Unsecure" icon on the URL bar. Turns out that some resources (the Stackoverflow widget) were being fetched from HTTP, so I just edited those to use their HTTPS coutnerparts (thankfully available) and I got the green padlock :)
When first installing it I forgot to add both domains to the initial
certbot command. I had to use the
\--expand argument in the same command to add the extra domain.
Also, note this bug about using underscore in subdomains. You won't be able to use Letsencrypt if your subdomain contains an underscore!