Go through the hallways until you reach a large room with a red button and a conversion gel tube. There's only one thing to do: press the red button. Doing so will make some bombs hit the floor out there. Put a portal on the ground below the bomb tube. Then put a portal in the middle of the white wall across from the conversion gel tube. Then press the red button. Conversion gel will coat the floor.
Make a portal on the floor, then make a portal of the other color on the wall of the area out there where the bombs were falling a moment ago. Go into the office and go through the exit door.
Up ahead, drop down onto the conveyor belt. Wheatley will soon make it start moving. Before you reach the end, step off onto the walkway with the red button on it.
Put a portal under the tubes where the bombs will drop from. (Press the red button if you're not sure where that is.) Put a portal of the other color under the orange gel tube. Then press the red button. The orange tube will shatter. Put a portal in that area so the orange gel falls into it. Then make another portal on the wall at the far end of the conveyor belt. This will coat the conveyor belt with orange gel. Now you can run the other way on the conveyor belt quickly enough to reach the portal at the end. Once you go through, go through the door next to the orange gel tube and go down the hallway.
Make a portal on the wall at the end of the narrow hallway here. Go back and put a portal of the other color under the orange gel tube if there isn't one already. The narrow hallway floor will become coated with orange gel. Now put a portal on the white floor under the light funnel. Run across the orange gel through the portals to end up in the light funnel.
When you reach the end of the funnel, walk out of it onto the walkway below. Go up the stairs and through the exit door. Look through the metal mesh above you and make a portal on the wall up there, then make a portal of the other color that you can walk into. Go through.