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 your own version of Coreboot onto your Chromebook (in-system). 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 at the moment, but flashrom should be working. Mario, Alex and ZGB are running Insyde H2C some UEFI.
|Release date||Manufacturer||Model||Project Code Name||Disassembly guide||Close cicuit via||Picture of Mainboard||SPI-Chip|
|May 2012||Samsung||Series 3 Chromebox||Stumpy||Jumper|
|May 2012||Samsung Series 5||550 Chromebook||Lumpy||Jumper|
|October 2012||Samsung||ARM Chromebook||Snow (aka Daisy)|
|November 2012||Acer||C7 Chromebook||Parrot|
|January 2013||Lenovo||Thinkpad X131e Chromebook||Stout|
|February 2013||HP||Pavilion 14 Chromebook||Butterfly|
|February 2013||Chromebook Pixel||Link||
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.