This FAQ answer the question "How to add Lets Encrypt to Apache on Digital Ocean Ubuntu 16.04?"

Let's Encrypt: HTTPS on Apache

Create a non root sudoer user

"sammy" is just an example of a user I have created on my server.

sudo usermod -aG sudo sammy

Look Up Digital Ocean Docs

https://www.digitalocean.com/community/tutorials/how-to-secure-apache-with-let-s-encrypt-on-ubuntu-14-04

Step 1: Execute steps to download and update the Let's Encrypt Client

Step 2: Execute steps to set up the SSL Certificate on just the Wordpress subdomain

I have installed Wordpress on my server and provided the domain name "wp", so I will just install the certificate on that subdomain.

 sudo certbot --apache -d wp.beckypeltz.online

I chose the redirect options. This means if the user types http://wp.beckypeltz.online, into the browser, I'll redirect to https://wp.beckypeltz.online. See the image below of this choice.