First of all when you go into Ulduar you'll see a big area with vehicles around it. You need to talk to the big NPC in the middle in order to activate them.
You jump on and the fight starts when you go down to the big area where the Storm Pillars and trash mobs are. You need to attack the pillars and the big mobs, pretty much ignore the dwarfs (your bikes can take care of them). Make sure to focus-kill the big giants, they hit hard.
We cleared 2 of the 4 elemental towers before getting to the repair pads - not sure if it has to be done like that or you can just clear to the pads and repair even before taking down the towers. Keep your distance from the tower while killing the big guardian spider as the combination of spider and tower will take your vehicle's life down quickly.
The boss fight itself is covered well in guides such as wowwiki. However I will say what I think are the keys to the fight:
- The Siege Engine which is not kiting should stay close behind the boss and use interrupt when possible. The turret operator should spend most of the fight firing anti-air ability on the floating pyrite ammo as this is where the DPS is.
- Demolishers should keep distance from boss and pick up any pyrite ammo which drops to the ground and use it - as I said, this is where the DPS is.
- The 2 people who get catapulted onto boss should read up on that tactic and be aware of what they need to do. It is not hard to do but it is very important, otherwise boss becomes too fast to get away from.