Searched and read the forums, read the wiki, and still do not feel prepared at all to flash the device.

I need a step-by-step guide
(One which hopefully can be used for other beginners needing help)
Would really like if someone were to help
(All I know right now is I do not want to reflash the eMMC)

First things first, what happens when you flash?
Everything is lost, even backups?

Guide please