Starting with FreeBSD 9.0, the default installer for FreeBSD has changed from sysinstall to bsdinstall. While there has been mixed reactions from old time users, the process of creating a custom install CD for FreeBSD has become a lot easier (although longer). bsdinstall uses a set of shell scripts which can be modified as per requirements.

First step is to install a fresh 9.0 RELEASE, and select the src to be installed along with the base system. Once the system with source is installed (by default /usr/src) execute the following commands

# cd /usr/src
# make buildworld buildkernel

Installation scripts are located at /usr/src/usr.sbin/bsdinstall/scripts/ . First file to run is ‘auto’, which then calls other files. Almost all of them are shell scripts. Remember, you do not need to repeat ‘make buildworld buildkernel’ if you are just making changes to the install script file.

# cd /usr/src/release
# make release

Once the release has been built we now copy the iso / usb image / ftp files to the desired directory

# make install DESTDIR=/usr/freebsd-snapshot clean

This creates the iso file, memory stick image and ftp folder for ftp install. In case you are wondering how to use the memory stick image, see this blog post:
http://koitsu.wordpress.com/2009/11/03/writing-freebsd-memstick-img-to-a-usb-drive-in-windows/

Cheers
Amitabh