The door in front of you won't open all the way. Create a portal beyond the door, and a portal of the other color that you can walk through. Go to the end of the hall. Follow Wheatley.
Soon all the lights turn out. Wheatley eventually turns on a flashlight. Follow him. He leads you to an inactive conveyor belt. Get onto it and continue following him to a set of stairs. This is part of the manufacturing area.
Soon Wheatley will tell you to wait. The walkway is broken, so jump forward to safely reach the other side down below. Continue along and you have to jump down to a moving conveyor belt. After a while, some robot arms will move panels into your path. Don't let these knock you off. There are also some robot arms doing laser etching in the panels below you. Don't walk into their path.
Wheatley will tell you when to get off of the moving conveyor belt. Continue along more metal walkways. Follow his light as he leads you through the cube tubes. Later he will say that you have to split up, and he tells you to portal up to the passage that he's shining his light on. Create a portal on the wall up there, then another portal on the wall nearby. Jump over to the metal walkway where you can get into the nearby portal.
You'll end up in a room. Go over to the opening in the wall. Wheatley shines his light on a distant wall. Create a portal there, then create a nearby portal and go through it. He'll show you another wall farther along. Create a portal there and a nearby portal and go through.
Make your way along the walkway until you get to a spot where the walkway is broken and there's a large area below. Carefully drop down from the broken walkway to a ledge below that's sticking out. Turn around and look for another ledge below you, then drop down to it. From there, go onto the cube tube and walk downward on it. At the bottom, you'll see the door to the factory. Drop into that area and go through the door. There are a couple of automatic doors. Go up to the second one to trigger the loading screen.