the more simple procedure (example base on certificate class 1 in startssl.com) is:
- create certificate in ispconfig
- take the field SSL Request content and do the certificate with this in startssl site
- take the content of certificate create and copy in "SSL Certificate" and take content of sub.class1.server.ca.pem and ca.pem and copy in "SSL Bundle" on ispconfig and select save option
Finish and work, sorry if i not explain good^^''