Dependencies: bsdtar(app-arch/libarchive), mkisofs(app-cdr/cdrtools), sys-fs/squashfs-tools
In this case we open and edit a Debian based LiveCD
Create directory for extract iso
mkdir $HOME/livecd
Extract ISO content to created directory with bsdtar
bsdtar -C $HOME/livecd -xf LiveCD.iso
Find the place where the filesystem squashfs is in this chace $HOME/livecd/live/filesystem.squashfs and extract it.
unsquashfs live/filesystem.squashfs
Now apply changes to extracted directory (./squashfs-root)
Create a new squashfs which contain changes
mksquashfs squashfs-root/ filesystem.squashfs
Finally create a new patched image of the LiveCD
mkisofs -o LiveCD_patched.iso -r -J -no-emul-boot -boot-load-size 4 -boot-info-table -b isolinux/isolinux.bin -c isolinux/boot.cat $HOME/livecd