This section is pretty straight forward. Just start following the path. When the Mako gets to what looks like an S turn there will be a Geth greeting party, which includes a Colossus. So greet them guns blazing.
Continue on the path. Continue around the bend. Before the next bend in the road will be some Geth Troopers.
Continue to follow the path around the S turn and down the long stretch. At the next bend will be some Geth Troopers.
Continue to follow path. At the end Shepard finds the Conduit. A cut scene will begin of Sovereign attacking the Citadel. When you have control of the Mako again Shepard has 40 seconds to use the conduit before it closes. Just drive right past the Geth and run into the Conduit. The Mako and all aboard will be sent to the Citadel. Cut scene will begin showing the Mako using some Geth as shock absorbers. This is the start of the last mission.