You first need to get through the chainlink fence. Either build a stack of crates over the chainlink fence, or throw a gas cylinder at the padlock to break it open. Once you are through the gate, enter the ventilation shaft.
When you fall, jump to the desk, across the boxes, and onto the stairs. Head upstairs, and look for the fuse box. Remove the fuse and put it in the right slot. Return downstairs, and collect the portal key.
In the fuse room, move the shelf, and enter the vent. Take a left turn at the vent and press the button to open the door.
Use the boxes to build a bridge across the electrified water, and cross it.
Take a right turn first, and enter the farthest room. Take the circuit boards, and place them into the computer on the wall.
There is also an artefact found on top of the furniture on the far side of that room.
When all circuit boards are in place, go through the vent in the south wall, and look for a canister of nitroglycerine. Bring it outside.
Enter the left room, and connect the power cable between the two plugs. Press the button to activate the moving platforms.
Take the nitroglyerine, and continue north. Collect the portal key on the way, it is located on a platform left of the other side. If desires, you can attempt to run along the cables holding the platforms - if you fall, you and your nitroglycerine will be teleported back to the junction with no ill effects.
Walk down the shaft, and bring the nitroglycerine to the cave-in. Find the fuse in the shaft, and connect it between the explosive and the oil pool. Pull the switch to ignite the oil and blast way the rocks.
Turn left, and look for the last portal key. You can now exit the level by going through the portal.
Before you finish the game, you have to choose whether to activate the furnace or to leave the room. This affets which ending you see.