There's a fire nearby. Look to the left to it to see another area. Go through it. Eventually you reach a dead end where there is a white wall that you can walk to, and another white wall on the other side of a chain link fence. Make portals on those walls and go through.
After going through, there is a pile of debris blocking the path. Past the debris, you can see a white ceiling. Create a portal on it and another portal of the other color on the nearby white wall. Go through.
Go through the gap in the tall wall then turn left and continue forward. There is a white wall straight ahead. Create a portal on it, then look to the right to see a light shining on another white wall. Make a portal of the other color on it and go through the nearby one.
There is a large area ahead of you. Drop down to the lower level. Walk along the straight metallic thing in the floor, then turn left and go up the slope toward the area with all of the "do not enter" and "keep out" signs. When you reach the wall, turn around and look for a white wall nearby. Walk to it and create a portal on it. Then turn around and look above the wall. A light is shining on a white wall up there. Create a portal and go through to get up there.
Go down the stairs and look for a "keep out" sign down below. There is a white wall there. Make a portal there, then go to the end of the path that you are on and create a portal of the other color on the wall there. Go through.
Look down at the area where there are more "keep out" signs. Drop down to there. Walk along this area until you reach a lever. Pull it. Go through the door that opens. Go over to the big hatch and go up the stairs. There are two rooms to either side, labeled 1 and 2. Use portals to go into room number one. Then go slightly outside of that room and create a portal on the wall next to room number two. Go through and create a portal behind the button in here so you can go through, press the button in room 1, then quickly press the button in room 2 before the countdown finishes.
Go down to where the hatch was removed. Press the button to open the door. Go forward and there will be a loading screen.