Ghost 01: Another Ghost can be found near the start of the game, going back as far as you can in Patrol mode. Look for a Ghost up in the ceiling, he can be pretty tricky to spot!
Ghost 02: As you enter the large room full of Fallen, head underneath the catwalk on the left-hand side of the room to find a ghost underneath where you came into the room.
Rounding the corner, you'll run into trip mines. Crouch to go under them and carefully fight your way past the next group of Fallen. The larger room beyond will send another wave of enemies your way. Use cover and pick them off as they come at you.
With the Fallen cleared, head out the room and through the spinning fans outside
Once outside you'll be treated to a wide open area with old tanks, spider walkers, and shortly - lots and lots of Fallen. As you venture out a flare will go out, signaling a dropship to unload a bunch of enemies in your location. Battle your way through them to the far left of the area.
"Ghost 03: At the far end of The Divide, to the right of where you exit The Breach, there is a large hollow cylinder. Inside, there is a dead Ghost on the ground.
After taking out more Fallen in the small rooms, you'll find a jumpship being worked over by several enemies - including a boss: Rahn, Devil Captain. This guy as a shield, so try using your grenade to punch through it before finishing him off yourself. With all enemies cleared, your Ghost will bring the jumpship back to working order, and send you off into orbit, completing the mission.