You can do it all in Webmin for the GUI minded users.. I wrote up a quick how-to about how I did it.. Hope it helps someone
Goto Webmin (usually https://<address>:10000)
In the console open Webmin - Webmin Configuration - SSL Encryption and click Certificate Signing Request tab.
Fill in the servername FQDN (ie efa-3.domain.lan), Organization, Country code, etc. and click Create Now
Copy the CSR text into a text file and send it to your CA. If all is well you should receive a certificate file in return.
Copy the text inside the resulting certificate file into your clipboard.
In the SSL Encryption part of Webmin click the Upload Certificate tab and paste the text in the Certificate text box and select "Entered below"
If your CA also requires you to install intermediate certificates you should copy and paste the text of that certificate into the "Chained Certificate text" and enable the "Entered below" for that option. I didn't have to do that as I used my own CA to generate the certificate.
When you press "Save" it will tell you where it saved the key and certificate files.
For instance: "The supplied SSL key has been saved in /etc/webmin/miniserv.pem, and the certificate in /etc/webmin/miniserv.cert." Remember/copy these locations for the next part!
Goto Servers-Apache Webserver. Select the Virtual server with port 443 in the Existing Virtual hosts tab. Select SSL Options.
Using the example above: put /etc/webmin/miniserv.cert in the Certificate/private key box. Put /etc/webmin/miniserv.pem in the Private Key file box. Replace these values with the ones from the previous step.
Go back to Servers - Apache Webserver and click Apply changes (upper-right corner)
Check your website on HTTPS and check the certificate, it should display (and use) the newly added certificate.
ps. I only did this for Apache, but if you need the same in Postfix it probably requires the same steps but using Servers - Postfix - SMTP Authentication and Encryption instead.