In essence you're just following markers in this part. Cortana will mark where each of the switches is. You run up to level 3, hit the first switch. Run back down to the ground floor, shoot a rocket launcher (or lob a grenade) into the marked hole. Run back up again, run back down again. You have to do this four times. It's really not that hard, and when you're done with all four, run right back up to level 3 again. That's where you're going to be leaving the level. Go out through the marked door and you'll take a lift up.

Halo Walkthrough

Halo 2 Walkthrough
Halo 3 Walkthrough