After you fling yourself through several portals, you reach Test Chamber 13.
Get a Cube
Create a portal on the ledge near the cube, then create a portal of the other color nearby, and take the cube through it. Put the cube onto the button, then go through the portal to reach the ledge and go through the chamber lock.
Go down to the lower area. Make a portal on the ceiling where the pellet containment machine is shining its orange light, then make a portal of the other color on the wall with the dark spot where the high energy pellet is hitting it. An Unstationary Scaffold will start to move. There is a cube on it.
Create a portal on the ceiling above where the scaffold is moving, then create a portal of the other color nearby, and walk through it when the scaffold is beneath the ceiling portal.
Push the Buttons
Take the cube and go down to the floor, then create a portal on the ceiling where there is a label with one dot on it (the label doesn't prevent the creation of a portal on that spot). Put the cube through the portal so that it falls onto the red button.
Next, make a portal on the ceiling where there is a label with two dots on it. Create a portal of the other color and go through it to land on the red button. The chamber lock will open. Create a portal on the wall beyond the chamber lock, then a portal of the other color nearby, and go through it.
Go into the elevator to go to Test Chamber 14.