[Sim]

G'day guys,
One question, how can I make a mod for myself?? I've downloaded tons of realism mods, some are really good and some aren't as real as others. But I was wondering if it were possible to make a mod for myself so I can make my single playing game to my preferences?
What do I need to do that? Is there a file I need to find?
If anyone could get back to me on that I'd really apreciate it,
Good stuff.
Sim