At this point Shepard is free to go where he wants. Zoom all the way out of map.
Choose Liara Dig Site. There are 4 systems here. Shepard can search each one until he finds the right one or just choose the Knossos system. Now choose the planet Therum. Next you will have to choose a party. Choose whoever you like.
A cut scene will show the Mako being dropped on the planet.
Just follow the path towards the Refinery. Along the way a ship will do a fly by and drop some Geth Armatures off. Kill them. Then proceed to the refinery. As you come to the Refinery someone will say itís too well defended. Take the side route and there will be only 1 turret instead of 3.
It's pretty easy to kill things in the Mako. Simply turn so that the Mako's side is facing one of the Armatures. Now you can quickly move to the left or right. When bullets come near you simply move out of their way. You can very easily keep your turret on any enemies while avoiding their slower bullets. Alternatively you can just hit your boosters and jump over the enemiesí bullets. I find it easier to move side to side.
After the turret is the Refinery compound which contains various Geths. There will be Geth Destroyers and a Juggernaut. Shepard can get out and kill them on foot, run them down with the Mako, or shoot them with the Mako.
Either way after the fight, search the buildings to the left but don't use the gate control. In the buildings to the right of where you come in should be the gate control you want to use.
After the gate is open continue down the path. Kill any Geth Armatures. At the tunnel Shepard will run into a bunch of Geth Rocket Troopers, Shock Troopers, and a Colossus. The Colossus is the toughest.
After the battle enter the tunnel. At the end will be some Rocket Troopers. Follow the path to another tunnel. Inside will be 2 Rocket Troopers. At the end will be a couple more. Keep following the path to the end and bail out of the Mako. Enter the next area on foot.