You know how I learned? Buy breaking stuff...
Seriously. Either breaking it or taking it apart. I remember many an afternoon when I had my old Honda in a pile of parts in the driveway for no really good reason. When I put those green LED's in my door handles, it was a matter of figuring out how the door panel came off, then where to bogart power from, etc, etc. I'd say what you really need is just a basic electrical knowledge (pos/neg, etc...), basic mechanical sense, and a lack of fear.
j/k
