This is a nice, concise guide. I use LetsEncrypt for one of my sites running on a DigitalOcean box, they have some excellent documentation there as well.
An alternative to LetsEncrypt is to sign up for Cloudflare's free tier. You get free SSL (using their shared certificate) and a bunch of other goodies. I have a couple of sites hosted on GitHub Pages using Cloudflare, so I have SSL and hosting at zero cost. Hard to beat that price!