Probably the easiest way to install secure, free and browsers trusted certificate for your site is Let's Encrypt certificate, so we strongly recommend using this method if you don't want to buy certificate from trusted authorities.
To enable Let's Encrypt, choose "Free & automatic certificate from Let's Encrypt" radius.
Enter Common name and E-mail.
Note: do not use "www" or "http(s)????/" in common name section.
Choose which Certificate Entries you want to secure. You can select maximum of 20.
Note: Always select yourdomain.com and www.yourdomain.com entries.
Press "Save" button. That's it!
Note: You may need to wait a few minutes for users to see your site as secured.