How to create a custom ISO image in CentOS

1. Create a directory to mount your source.

   mkdir /tmp/bootiso
   

2. Loop mount the source ISO you are modifying. (Download from Red Hat / CentOS.)

   mount -o loop /path/to/some.iso /tmp/bootiso
   

3. Create a working directory for your customized media.

   mkdir /tmp/bootisoks
   

4. Copy the source media to the working directory.

   cp -r /tmp/bootiso/* /tmp/bootisoks/
   

5. Unmount the source ISO and remove the directory.

   umount /tmp/bootiso && rmdir /tmp/bootiso
   

6. Change permissions on the working directory.

   chmod -R u+w /tmp/bootisoks
   

7. Copy your Kickstart script which has been modified for the packages and %post to the working directory.

   cp /path/to/someks.cfg /tmp/bootisoks/isolinux/ks.cfg
   

8. Copy any additional RPMs to the directory structure and update the metadata.

   cp /path/to/*.rpm /tmp/bootisoks/Packages/.
   cd /tmp/bootisoks/Packages && createrepo -dpo .. .
   

9. Add kickstart to boot options.

   sed -i 's/append\ initrd\=initrd.img/append initrd=initrd.img\ ks\=cdrom:\/ks.cfg/' /tmp/bootisoks/isolinux/isolinux.cfg
   

10. Create the new ISO file.

    cd /tmp/bootisoks && \ 
    mkisofs -o /tmp/boot.iso -b isolinux.bin -c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -V "CentOS 7 x86_64" -R -J -v -T isolinux/. .
    

11. (Optional) Use isohybrid if you want to dd the ISO file to a bootable USB key.

    isohybrid /tmp/boot.iso
    

12. Add an MD5 checksum (to allow testing of media).

    implantisomd5 /tmp/boot.iso
    

If you need more help creating the Kickstart script, I suggest starting with the official Red Hat documentation.