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 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 anyway).
Note: The first three Chromebooks are not supported by coreboot (and flashrom!?) at the moment, but flashrom is working. Mario, Alex and ZGB are running Insyde H2C some UEFI.
General Hardware Preperation
Use the disassembly guides to disassemble your Chromebook till you see place were you can put on the write-protect-jumper or srew in a Bolt. Do either. For example In Lumpy and Stumpy you have to put a jumper on place between the Battery and Ram next to the Keyboard-Connector. Now Assemble again (follow the guide). Now you are ready to flash your own Coreboot-version onto your Chromebook.