How do I backup VZW ROM?

[SysWiz]

Newbie alert. I am fairly new to the whole PPC scene. I re-flashed my previous Razr phone from VZW to ALLTEL to get around some of the stupid VZW limitations, so I'm not new to mucking around with my phone. This time, I want to jump in and try some of the custom VZW ROMs out there, but I'd like a tried-and-true way to backup my ROM and be able to restore it. How do I backup my existing ROM for testing? I've seen the stock VZW RUU file out there, but I'd like to know how to create my own - just to be safe. Thanks!