I don't think there is a "how to" for it for the ZS but there are some videos out there for other brands.
It's a job on my list, but mostly so that I can build a custom airbox at the front as I had a few bits chopped out the front to get the single pass IC to fit.
Not sure the gauge, but you want to it to be pretty high & battery cable spec - better to over spec than under.
Seatback bolt could be a good earth - I'd probably not want to use the rear light area just because I'd not want to increase the risk of any potential shorts *in case" scenario.
It does depend on where you plan to put the battery as to whether you put your own earth point in as well - if you're mounting the battery tray and cutting some of the boot matt down, you can put your own earth wherever is suitable really
