The rotation of a Frost Mage consists in the following priority list, assuming you have taken Frost Bomb Icon Frost Bomb as a Tier 5 talent. The most important thing about Frost Bomb is that you are not required to keep it up at all times, but only when you can make use of it (i.e., when you are certain to be able to cast at least 2 Ice Lance Icon Ice Lance with Fingers of Frost Icon Fingers of Frost). See below the list for other talent options.
Apply Frost Bomb Icon Frost Bomb or refresh it (if it is about to expire), but only if one of the following condition holds (let it drop otherwise):
you have 2 charges of Fingers of Frost Icon Fingers of Frost or;
Frozen Orb Icon Frozen Orb is about to come off cooldown;
Water Jet Icon Water Jet is about to come off cooldown.
Cast Frozen Orb Icon Frozen Orb on cooldown (unless you want to save it for an upcoming AoE phase).
Cast Freeze Icon Freeze from your Water Elemental (never use it while Frozen Orb Icon Frozen Orb is active, though).
Bosses are immune to Freeze, so only cast it if there are adds you can root. Doing so will give you 1 charge of Fingers of Frost Icon Fingers of Frost (2 if it hits more than 1 enemy).
Cast Ice Lance Icon Ice Lance when Frost Bomb Icon Frost Bomb is up and you have 1 or 2 charges of Fingers of Frost Icon Fingers of Frost. Otherwise, let Fingers of Frost Icon Fingers of Frost build up until you have 2 charges, so that you can apply Frost Bomb, before consuming your charges. Only cast Ice Lance without Frost Bomb, if you have 1 charge of Fingers of Frost and it is about to expire.
Cast Frostfire Bolt Icon Frostfire Bolt when Brain Freeze Icon Brain Freeze procs (you can have up to 2 charges). Frostfire Bolt can give you a charge of Fingers of Frost, so avoid casting it if you already have 2 charges.
Cast Water Jet Icon Water Jet from your Water Elemental.
Your goal is to cast Frostbolt Icon Frostbolt twice while Water Jet is being channeled to generate charges of Fingers of Frost Icon Fingers of Frost (see our Water Jet section for more information).
Never use Water Jet while Frozen Orb Icon Frozen Orb is active.
Spend your charges of Fingers of Frost Icon Fingers of Frost before using Water Jet.
Cast Frostbolt Icon Frostbolt as a filler spell.
Water Jet Icon Water Jet and Freeze Icon Freeze share a cooldown. In a raiding environment, you should always give priority to Water Jet, unless you need the root effect or you can get a good Shatter Icon Shatter off on a number of adds before using Ice Nova Icon Ice Nova on them (if talented).