Here is a example of how to set up one IP alias and help to create a new one easily.
In the IP alias dialog, define a full range of IP aliases. This will likely be sufficient for quite some time. Suppose your eth0 network is 192.168.1.0 and there is an unused section. Just enter
which will allocate 51 IP aliases for future use.
Now, in the "Configure domain name server" menu, you select the "IP allocation space" entry. Enter the range and a small description such as "Virtual domains." The DNS is not always on the same machine where you define the IP aliases.
Go into the DNS and add a host for the domain, generally www.new_domain. For the IP number, hit ctrl-X and Linuxconf will compute the first unused IP number in the IP aliases range. Pick this number.
Then go into the httpd configuration and add a section for the new www domain. Repeat this step for each new domain until you are out of IP aliases.
Using this strategy (defining a bunch of aliases first, telling the DNS about them next, and using them last), you will avoid many errors.