Becoming a raid-level tank is a multi-step process that can be quite daunting at first glance. Most people find this transition from casual tanking to raid-level tanking quite difficult, but I have some tips that might help:

1) Do extreme primals. They're great practice!
2) Maximize your DPS. Late game raids right now (Alexander Midas Savage) are full of DPS checks.
3) As long as you have the minimum ilvl for the instance, you should be good to go. Of course, higher ilvl will make things a lot easier and will boost your dps.
4) Join a "newbie static". That is, join or make a static with people who haven't yet done any lategame raids but who wish to start doing so.
5) Parsers can help with raid analysis (dps, incoming damage, etc etc)
6) Gear up as fast as possible once you're raiding.

Watching guides are necessary. It's up to you as to how much you want to study up.