At the beginning of chamber 4-11 put a portal through the gap on the right-hand wall, and the other directly in front of you. After going through the portal look up to your left to see a grate and portable wall. Place a portal here and the other on the tilted ramp just before you.
Once up top, look to your left to see another tilted ramp. Place a portal there, and then jump off the ledge into your original portal to be launched across the gap. Place a new portal in front of you and one to the left on the new tilted ramp. Once on across the gamp on the grated ledge drop down and look to your right to find a button.
Press the button to drop a blue goo-covered cube, and press the button 15 more times until the cube stops spawning with blue goo.
the video guide starts at 0:57