Installing SSL is very easy. All you need to do is be in your network view and click on "Actions" and then "SSL Certificate." There you can install Let's Encrypt (free SSL certificate) or a custom SSL certificate.
If you wish to install a custom SSL certificate, you can generate a CSR if you don't have one already.
SSL installer runs in 15 minute intervals and your domain's health status must be "Healthy" before you can install a SSL certificate.
If you install a SSL certificate when your domain's health is in "Attention" or "Error" state, then SSL will not be installed before your domain's health turns to "Healthy."
Also, make sure your domain don't have AAAA (IPv6) record set up for your domain. Let's Encrypt will use the AAAA instead, and the SSL installation will fail.