1) Enable SSL in your ISPConfig website and hit save.
2) Go to the new SSL tab in the website settings in ISPConfig, enter the certificate details and select create as action and click on the save button.
Now you will have to wait about a minute.
4) Go to the SSL tab again, now there is a certificate signing request in the one field, use this request to create a certificate a cacert.org.
5) enter the certificate code you got from cacaert.org in the certificate field on the ssl tab in ispconfig, select "save certificate" as action an click on the save button.