First, go to your Plesk domains page where you want the SSL certificate installed. This will look like the image below.
Near the middle of that center panel you'll see a sort of lock icon with 'Let's Encrypt' next to it. Let's Encrypt is an SSL provider which lets you set up an SSL certificate for free. Simply click that button to continue to the screen below.
Your email will already be filled in with the same one you used when you signed up. You'll receive emails when the certificate is automatically renewed, or if there are any problems with it. If you want to use www in your domain URL, you should probably check the box to secure it as well.
Once you click 'Install', the certificate is issued, installed, and set to renew automatically. That's it! Your site will now be secured and safe to browse on =)
