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. Midway through the air, place your portal on the tilted ramp before you and pass through this loop about six or so times.
The video guide starts at 0:13