Instructions
Start "The Ark" mission from the beginning or the Alpha checkpoint.
Proceed through the mission until you encounter the Scarab.
Kill all of the Covenant forces in the area. You need to avoid destroying at least one Ghost, the single-person Covenant vehicle.
Kill any remaining UNSC Marine allies. You cannot drive the Scarab if any are still alive. The AI of the Scarab attempts to follow the player if the Marines are dead. This is needed to control it.
Board the Ghost.
Drive the Ghost up one of the ramp-shaped rocks to drive it on top of the Scarab. In most cases, the Ghost's engine boost is going to be required to do this. This is the most difficult part. It is easy to overshoot or undershoot the top of the Scarab.
Get out of the Ghost.
Walk to the top of the "head" of the Scarab. This is the lower half of the body.
Walk to the left or right side of the Scarab's head. The AI of the Scarab attempts to follow you, causing the Scarab to turn in the same direction. This allows you to control the Scarab's movement.