It's largely from having played the game as long as I have, but I just sort of know the rules. Half the time, we don't even bother having a rulebook on hand-- we all know the to-hit chart, the vehicle damage tables, and all that fun stuff, and we know the game by heart, so on the odd occasion something comes up, we just sort of fudge it and worry about it later. Most of the time, we followed the rules unknowingly.
If you're going to ask, "Gee, how'd you learn the rules to that level?" my answer is thus-- first, play the game for most of a decade. Second, sit down with the rulebook, just as if it were a textbook for school, and study it. Give yourself quizzes. You don't have to know it perfectly-- I'd shoot for knowing about 85-95% of the rules section, which isn't that long, really. It's mostly pictures. Get some spare models, set up the diagrams, and look at it that way, rather than just looking at the pictures-- somehow, it's a better learning tool. Spend some time playing games. There's a difference between playing a game and fighting a battle, so to speak-- in one instance, you're just screwing around with your buddies, and it shouldn't matter in the slightest whether you win or not. Have a drink, relax, and try to memorize the rules as you play. In the other, you're trying to claim that cash prize for a three-round tournament.
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