Certbot is packaged in EPEL (Extra Packages for Enterprise Linux). To use Certbot, you must first enable the EPEL repository.
After doing this, you can install Certbot by running:
sudo yum install certbot
Certbot supports a number of different “plugins” that can be used to obtain and/or install certificates.
Since your server architecture doesn’t yet support automatic installation you should probably use the certonly command to obtain your certificate.
certbot certonly --webroot -w /var/www/example -d example.com -d www.example.com -w /var/www/thing -d thing.is -d m.thing.is
This command will obtain a single cert for example.com, www.example.com, thing.is, and m.thing.is; it will place files below /var/www/example to prove control of the first two domains, and under /var/www/thing for the second pair.
Currently it is possible to perform a DNS validation also with the certbot LetsEncrypt client