How to you create your own action replay codes?

By this I mean, how do you create your own codes. I know someho hex editors are involved (or so I've been told) and I know how to use them SOMEWHAT, buut... how the hell do you tell what all of those values mean? How does one know what effects what inside a game? No I don't play pokemon so please don't refer me to pokesav unless it does things for games other than that. I want to create AR codes for gamecube games, Wii games, and DS games. If there are different programs for each, please refer me to them :)