A good place to start is the Chromium site. They have guides howto set your Chromebook into developer-mode and disassembly guides for almost all Chormebooks.
Both things will be nessary, if you want to flash (in-system) your own version of Coreboot onto your Chromebook. It is nessary to close/open a circuit on the motherboard via either a jumper or a srew to disable the write-protection of the spi chip. And to use the flashing tool flashrom in ChromeOs, you have to be in the developer-mode. If you already installed a Gnu/Linux-system or likely you can also use the flashrom from there (but then you would be in developer-mode any way).
Note: The first three Chromebooks are not supported by coreboot (and flashrom!?) at the moment. Mario, Alex and ZGB are running Insyde H2C some UEFI.
are running Insyde H2C some UEFI.
Lumpy & Stumpy
Use the disassembly guide to disassemble your Chromebook till you see the write-protect-jumper between the Battery and Ram next to the Keyboard-Connector. Pull it or if there is non put one on. Now Assemble again and flash your own coreboot version onto your Chromebook.