Ok, I understand that red, green and blue colors control different things in the shader, but how exactly do I make a multipurpose map? Do I just color onto it or do I make different parts in the channels black or white depending on what area should be red, blue or green?

Just add more detail in the texture such as scratches, shaded areas and such in the multi. This will show up as reflection (or lack of) and will make the actual diffuse texture appear more enhanced.