Chamber 19 is the final test, after which you have been promised cake.
Welcome to the final test. When you are done, you will drop the device in the equipment recovery annex. Enrichment Center Regulations require both hands to be empty, before any cake...
—GLaDOS
First room
The first thing you see in the room is a pellet emitter, a pellet receiver and two platforms in a 45o angle to the floor (as seen in the overview image). The first thing that should be done, is placing a portal below the pellet emitter. Remember, not to create another portal, as the pellet would go through it. After this has been done, make ready to shoot at the other platform (timing is all important here). When the pellet is between the platform and the wall (as in the image on the right), make a portal below the pellet receiver. The pellet receiver then starts the moving platforms.
Now, create a portal behind the platform you can see, while looking down the hallway, and create another accessible portal.
Hallway
On the platform, you can see an appendix area farther down the hallway: Create a portal here, and go through the one you just exited, so you are now in the appendix area. In this area there is a button, which opens the door next to you for a few seconds. Push this button, and create a portal on the other side of the door. When a platform passes the portal, walk onto it (You do not have enough time to go through the door on a platform). In front of you there is a pellet ricocheting between the window to the first room, and the wall of the hallway. In order to deal with this, you have to create a portal on the wall, and when the pellet has gone through it, quickly create the other portal another place, so the pellet does not ricochet back through the portal.
Now you are facing a wall, which you have to cross by creating a portal on the wall, and another on a higher ground on the other side, as shown in the picture to the left. When the platform reaches the wall, go into the portal, and jump back onto it when it has passed the wall.
Oven
At this point you find out, that the cake is, as prior wall-writings have indicated, a lie (Or at least going through the fire is not the way to get it). Therefore, you have to get to the platform above the oven. This is done by creating a portal on the wall behind the platform, and a portal on the wall to your side (slightly below your level, as seen in the image on the right). When this has been done, walk through the portal.
Congratulations, the test is now over. All Aperture technologies remain safely operational up to a temperature of 4000 degrees kelvin. Rest assured that there is absolutely no chance of a dangerous equipment malfunction prior to your victory incandescence. Thank you for participating in this Aperture Science computer aided enrichment activity. Goodbye
—GLaDOS
On the platform, create a portal positioned as high as possible on the wall, and then one accessible to you. Go through the portal, and as you are falling to the ground, create a portal on the spot you will land on.
Fourth room
As you try to go up the stairs, these will fall down. Create, instead, a portal on the second level and one on your level, and go through them. Or just jump onto the stairs.
Fifth room
In the next room, you have to get past a locked grid-door. Do this by creating a portal on the other side of it, and one on the wall, accessible to you.
Sixth room
Following the hallway, you come to a room with two fans blocking your advance. Create a portal on the other side of one of the fans and one on the floor in front of your. Go through the portal.
The room itself has a floor of acid, which you have to cross, by creating a portal on the opposite walkway, and one on your side, as shown on the image.
After this, go to the other end of the walkway, and create a portal in the upper far right corner, of the opposite wall (as shown in the image), and one beneath you, in order to get to the walkway in the middle. After this go away from the room (The door on the other side has no relevance).
Seventh room
Get to the higher platform, by creating a portal there, and another one accessible to you. Then, create a portal in the upper far right corner of the wall. Let yourself fall, and create a portal, on the spot you will land in. This way you will build up enough momentum to cross to the platform opposite to the wall.
What are you doing? You haven't escaped, you know. You're not even going the right way... Hello? Is anyone there?
—GLaDOS
Follow the hallway to the tube transporting cubes, and enter it.
Test area
At the end of the tube, you are put into a cube dispenser, from which you enter a test area.
In the test area, stand on the button in the middle of the room, and create a portal behind the door that opens. Then create an accessible portal, and walk through it. When walking farther, you get to the chamber lock, however, this is malfunctional, and you have to jump down.
Chamber lock area
Now, create a portal on the backside of the platform on the other side of the grid-wall (as seen in the image), and then one under yourself. Then duck, in order to get into the room.
Uh oh! Somebody cut the cake. I told them to wait for you, but they cut it anyway. There is still something left, if you hurry back.
—GLaDOS
When you are in the room, go to the two pistons, which raise to two different levels. Go onto the one that raises the highest, and create a portal on the top of the platform you just entered through. Go back under the platform, create a portal there and use it to get on top of the platform.
Walk onto the cylinder next to the platform, and let it push you to the platform in the platform to continue. Walk along the hallway, until you get to a grating. Create a portal on the other side of it, and go back to the platform to create a portal there, and walk through it.
Vent shaft
Create a portal as high as possible, opposite to the walkway leading away from the shaft. Then create one accessible to you. On the other side of the portal you have to jump in order to make it to the other side (the timing here can be rather hard).