Learn by blindly tweaking the rules... that's probably the best way. It's the original way, the oldest way, the way I did it and also the way Cannis, Deezire, Mooman did it etc. etc. In fact I had never even heard of Deezire's Modding Encyclopaedia until AFTER I finished version 1.0 of my mod.
The way I did things was I played Red Alert 2 (never had a concept of "modding" a game at all) and then I found out that some people (very few at that time) could actually change the gameplay to add new things... they released software called a "mod". I palyed a few mods and they injected new life into the game and I played it more. One day, I decided I would change something - better paradrops. I managed to find something called a "rules" fiule. I knew about ini files in general from editing control chemes from other games, but this rules file was by far the larges ini file I had ever seen. After searching through it, I found the paradrops. I made a small change, and placed the rules file in the game's folder (as most inis should go...). It worked! I remember how cool it was to see a change I made in a game. After that I just read that rules file, explored it for days, using many different tags, tweaking many things. It was easy because I loved it. Eventually I had changed enough things to make the game quite different. It was around that time I started looking to the internet for help on making other changes such as art, cameos, game modes etc. I made new cameos and changed the appropriate game strings.
What I had then was my own "complete" mod. Never intended to be "released" - it was jsut mine. Shortly afterwards though, I decided to let RADEN host the mod - I thought up a name, put together a readme (the longest readme EVER mind you) and compiled the thing into a neat zip package. That was how WR came to be.
Then I discovered Deezire.net (never visited C-GEN because it was unviewable unless you signed up

) and started becoming more serious about modding, starting my own site and planning a much improved version. That version is STILL unreleased...
...and that's my life story from the ages 15-20.