What is a good guide that shows new Android users how to properly backup and Flash a new ROM?
CyanogenMod has a good overview with many devices and a full update guide per device (Captivate as example here). Here is an explanation of commonly used terminology (fastboot, adb…).

There are some differences between manufacturers/brands:  


Google: very easy, with standard tools (adb, fastboot)
Sony: rather easy, all standard tools + special tool (flashtool by androxyde), locked bootloader that needs an unlock code from website (not possible for all branded phones)
Motorola: rather hard to mod, locked bootloaders
Samsung: rather easy, unlocked bootloaders (except for some branded phones)