Calum's link is the way I do it "after" I have a system installed, however, if you do not check the "Skip Bootdisk Creation" checkbox when you install RedHat it will create it before the reboot after the initial install.
The only issue you "might" have with the "mkbootdisk" command is if you have SCSI drives that require a kernel module to be loaded at boot time, in which case you must also create an "initrd" with the appropriate module (should find one already made in /boot after install) that needs to be included in the "mkbootdisk" command line. If you are running IDE drives this should not be required...
