The best way to figure out what to mark is to pay attention to what is in the pull. Casters are usually easy to pick out - you usually want those to go first, most of the time. Use skull and X to mark out first and second in the kill order, then triangle if need be. Box and moon are usually CC, moon being sheep/hex and box is hunter trap (in my groups anyway). Be sure to figure out what CC abilities you have and if it is easier to get the casters out of the way with those.
A lot of marking comes down to what is in the pull... and you're only going to know that through experience or from asking others that have done the run before. Get an addon so that you can mark quickly - http://wow.curse.com/downloads/wow-a...arkingbar.aspx looks pretty good, I'll probably get it myself (for when I get back into tanking seriously). Unfortunately there are a lot of pulls that have unique aspects to them, so it depends on your group and how you handle the sudden abilities that come up. You get it from experience, and making sure that you're paying attention. Go through your combat log if you wipe and see what mob used what ability, and if you think that was what was wiping you, point it out and mark it up.