Grab the Weighted Companion Cube and go downstairs. Place a portal on the white wall under the stairs, then enter in the main room with the Companion Cube.
Put the other portal on the floor and pass through it wihtout taking the Companion Cube. Open the door to the left, move forward and turn left.
Now, turn left again if you didn't carry the Companion Cube from the Test Chamber 18. Otherwise, pass through the door facing you.
Turn right, move forvard, open the door, turn right, turn left, go straight, open the door, go upstairs, turn left, open the door, turn right and press the Switch.
Place the first portal near you to go back directly in the main room. Fire a portal on the motionless angled panel and the other on the floor.
Start your Infinite Jump. Jump through the portal and, just before touching the ground, place the same portal as the previous under you.
Repeat this process in order to gain height. When you can see the white floor on the ledge in front of the door, shoot a portal onto.
Go through the portal. Press the Switch near you until the panel it links is angled to the left.
Now, place a portal on the bottom's floor next to the ledge. Wait for the angled panel on the right to move, then put the other portal onto.
Jump through the portal. You should reach the other ledge. Fire a portal on the angled panel at the end of the room, and the other under the Companion Cube.
Grab the Companion Cube and place it on the button. Push the Switch next to you until the panel it links is angled downward.
Press the Switch facing the High Energy Pellet emitter until the panel it links is angled to the right.
Push the Switch left of the white motionless angled panel until the panel it links is angled to the right.
Press the Switch right of the white motionless angled panel until the panel it links is angled upward. Now, pass through the portal on the floor.
Enter the little room after the door now open, then place the same portal as the previous on the wall left to the catcher facing the door.
Wait for the pellet to reach the catcher, then put the same portal as the previous on the wall left to the catcher left to the door.
Wait for the pellet to reach the catcher, then shoot the same portal as the previous on the wall left to the door.
Wait for the pellet to reach the last catcher, then go back to the button with the Companion Cube onto. Place the same portal as the previous on the other ledge's floor.
Grab the Companion Cube, go through the portal and put the Companion Cube on the button next to the door. Pass through the door now open.
Put a portal on the wallpanel marked with a one dot sign and the other on the wallpanel marked with a two dots sign.
Just after going through the portal, fire a portal on the platform marked with a three dots sign and the other on the floor marked with a four dots sign.
Jump through the portal and try to reach a platform upper than the one you just passed.
Wait for the platform where you are to move. Then, quickly, place the first portal on this platform and jump through the portal on the floor.
Repeat this process until you reach the top of the room. Put the same portal as the previous behind the room corner's ledge, then pass through the portal on the bottom.
