A pretty simple puzzle, you should have this polished off in just over a minute.
Look for the transportation beam above you and get the first portal on the chequered wall panel where the beam is hitting the wall. The second portal should then be placed on the central panel in the middle of the room. Now stand on the jump plate to get carried across to the beam.
Once you’re being pushed up against the roof, look forwards for the slanted wall panels on the left, you’ll want the original beam entrance portal fired on the slanted panel, this’ll cause you to drop and get carried across the pit towards the exit door.
Just like you done with yourself, fire the entrance portal on the slanted wall panel and the Cube will drop through the centre of the room and come flying out towards you. Catch it and set it on the switch to open the exit.
Once you’re by the exit, walk over to the red button, but before hitting it, get a portal back onto the chequered wall panel where the beam is (like before) and once again place your next portal on the middle panel in the middle of the chasm. Now hit the button and the beam will catch the Cube in mid flight.