In a typical environment, you can create a copy of httpd.conf and then modify the following properties on the new file.
# pid file
# http listen port
# log files
CustomLog logs/access_log combined
# server name
# document root for the default site
And depending on your implementation you may need to modify additional properties like LockFile (if you are running on a NFS)
And as always you will have to customize the virtual host definitions if you use them.
Controlling the new instance
Say for example if the name of the newly copied file is /etc/httpd/conf/instance1.conf, then you can start this new instance using the following command
httpd -f /etc/httpd/conf/instance1.conf -k start
Another useful option of the httpd command will be the -t option to test the configuration file for errors.
httpd -f /etc/httpd/conf/instance1.conf -t
See 'man 8 httpd' for more information on how to use httpd command.
And as others hinted, you could should create a separate init script to help you manage this instance. The stock RH /etc/init.d/httpd script should act as a starting point.