Specify services you want to start or one shot commands like changing mount options or tweak power policy.
A device running Chromium OS that has been switched to developer mode is needed.
Remove rootfs verification if haven't done this already. There is a helper script which you can execute by running # bash remove-rootfs-verification.sh which removes verification and keeps rootfs readonly by default.
Run # bash install.sh to install upstart task and create placeholder scripts.
Modification of the rootfs won't survive after update or if you switch partition branches (/usr/local persists as it's not a part of rootfs) so you will need to rerun the install script.
Treat it like rc.local in tradition linux distro. The example folder contains some basic examples which make life easier on chrome os.