Upon entering the Test Chamber, immediately fire a portal up at the top-left corner of the room (where the single wall panel is) and then another portal under the gel dispenser unit just to the right (on the floor).
The Propulsion Gel will significantly enhance your velocity up as you run over it, so stand by the portal on the ground floor and then run up the ramp ahead at speed. Once you’ve crossed the chasm, turn right around and zoom in to the top right-hand corner of the room.
Fire a portal at the panel you used to reach the button before and then walk back into the corridor you’re in and on the left is another shoot-able wall. Place your entrance portal here and step on through.
Note the colour of the portal directly above your head and place that directly under the Propulsion Gel dispenser again. Press the button and you’ll coat the corridor off in the distance. Drop back down onto the orange gel on the floor and run back up the ramp and into the corridor.
Run over the gel here and you’ll find you have enough speed to make it across the bottomless pit below you.
Pick the Cube back up and then step through the portal so you’re once again standing on the orange gel. Look back into the main room and you’ll want to place an exit portal on the wall near the red square switch on the floor (on the left side of the room).
Look for the square wall panel on your left (if you look up you should spot the small red button hanging from the ceiling), so note what colour the exit portal leading to the ramp is and use the entrance portal on the panel on the floor.
Step through the portal and - slowly - walk up the raised ramp, turn right around and run through the portal straight ahead. The momentum gained should see you soar right up to the button, so quickly press it then use the floor gel to run back up the ramp and to the newly opened exit.
MISSABLE ACHIEVEMENT/TROPHY ALERT!
After exiting the chamber, you’ll enter an open walkway. Note the closed gate to your left and the white wall panel sneakily hidden from full view just up ahead of the closed gate.
Fire a portal up onto this wall and then another on the wall just up and to your right. Walk into the office and look directly at the large wall portrait for a few seconds to get the 'Portrait of a Lady' Achievement/Trophy.