I doubt you were doing it wrong, the actual pulling is simple enough. It's mainly a matter of doing the instances over and over (ideally on normal as you're leveling, imho) so that you know which groups you can charge, which ones you can pull, which ones you can LOS and how they all interact with the nearby patrols.
It's still not too late to do that if there's an instance that keeps screwing you with bad pulls. Get some low-level guildies, or just pick up a low level group out of LFM; they'll be tickled to have a high-level tank. Tell them you're not going to zerg the instance super hard because you're refining your pull technique.
After that, it's just a matter of remembering where things go wrong and trying something different next time.

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