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.
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.