I think 2 things are needed:
1) an understanding of the concepts (e.g. what do compressors do and fundamentally how do they do it)
2) an understanding of the software in question (e.g. how are the delivered compressors in Soundforge different from those in Reaper when setup to accomplish the same result). Sometimes this is the same, sometimes it's not, depends on the task.
The limiter post you made is what made me think of the second one; I can talk about how a function works, but accomplishing that function is different in each piece of software or plugin.
a third thing that is needed in practice but not theory (compared to the first two) is a revealing playback environment and learning how to truly listen (what do various frequencies sound like, what does imaging sound like, etc). I have one which I'm now comfortable enough knowing where it's few flaws are that I can make decent decisions on it, but the other 4 options I have, while valuable, I wouldn't commit anything on. I use those others as test beds (does it sound good in the car, how about with earbuds, how about trebley speakers, lets try with a subwoofer, etc). That allows me to compare edits I've made, not necessarily decide what edits to make.
So where would you like to start?