Re: WHAT form of Linux ?
'simply' likely depends on the Chromebook.
My Lenovo Chromebook from a few years back (N20P), required disassembling to get at the main board, and the 'wrong' side of the board as well, so not just a one side cover removal . This included a somewhat unnerving 'cracking' open of the case. I'd done this with mobile phones before now, so had some experience, and the tools needed, but it still needed quite a bit of force!
Once done, there was a screw that needed to be removed, this had a very large flat head, that bridged two solder pads on the motherboard. Removing this, unlocked the Chromebook bios/flash area, essentially allowing you to replace the boot side of things, and so effectively changing it into a mostly standard Laptop/Netbook device. (Without this, things are basically temporary).
Once done, installing Gallium OS (based on Xubuntu) was as straightforward as sticking Ubuntu onto a standard Laptop would be.
Makes quite a nice little Linux netbook now!
Bonus was avoiding landfilling a perfectly good device due to Googles arbitrary fixed support lifecycle for Chromebooks. aka planned obsolescence.