Time to meet the Turrets! Knock the first one over by any means. You can either get behind the second one and knock it over, or portal under it to send it flying. Do the same for the next one.
Pop out for a second and shoot a portal above one of them and drop a cube on it. Do the same for the next one.
Portal behind the next two and knock them over. Be sure to visit the room with tons of cubes for ammo for the next room.
You can put a portal under one of the turrets, and the other two need to have a cube dropped on top of them. Put a cube on the button, but don't stand in front of the door, a turret is right behind it!
You can portal through this grid, to knock the turret over and send it through your portal. There will be another turret around the corner facing away, portal through the grid and know it over too.