How to change Betta fish water?

Aquarium pros, please help me change the water in my kid's fish tank for the first time.
A couple of months ago, beloved daughter won four "free" fish (grumble grumble $60 worth of tank and supplies for four "free" 99 cent fish) at a school fundraiser. Today, it's evident that John, Paul, George and Ringo need to get their tank freshened. I've never done this before, and I'm petrified that one or more of the little guys won't survive if I do something wrong.

So, in layman's terms, how do I do this, step-by-step? Are there any tips you can share, things you've learned over the years that make the process goof-proof? Do all of the rocks and the yellow submarine and all that need to be rinsed, or do we buy special chemicals, do we do something to the water first, etc.? Do I need to do something to the new water before putting them back in? Talk me through this, please, because my girl will be devastated if something goes wrong.