There is no difference for the part that has to be done in ispconfig. Just use the dns wizard and create the record like you do it for any other domain. This is described in the manual in detail.
The only thing that differs is that you have to create glue records at the domain registry (and not in ispconfig) for the nameservers. The exact steps differ depending on the company where you registered the domains, so you might want to ask the support from that company in case you dont know how to create glue records in their domain interface.