Fire off a portal at the nearest wall and you’ll fall into the test chamber properly. The first thing to do is to run over to the platform ledge straight ahead.
There’s a Companion Cube waiting for you in a clear room to your left, so aim through the hole in the roof and it’ll fall through to the other side of the room. Drop the Cube down on the red button and head through both sets of doors to the second half of this chamber.
From here you should spot the orange portal already open for you, so drop down into it and you’ll be flung across the chamber (where the Companion Cube is residing).
You’ll find yourself hitting a wall at the back of the chamber, so turn around and drop the Cube on the large red button on the floor (behind the broken glass panel to your left).
For the final time, drop into the orange portal below you and you’ll now land in front of the doorway leading to the exit.