1. Measure vent length and width.
2. Find the angle from the furthest edge of the AC vent to the furthest corner of the room. This may not cross the center but it will result in even distribution.
3. Using a 3 to 1 ratio -- (ex. 3 inch high vent to 1 inch opening) trace out a top and identical bottom length flap on the cardboard. Using the angle furthest, cut a length longer than the other to redirect the flow of air. Do not intend to reduce the opening too small bcz too little room for displacement will over work the A/C and reduce efficiency.
4. Using glue (hot or cold) affix the corners. Using tape, seal the edges.
5. Affixing to the unit, a proper seal around the edges flush with the vent will reduce escaping air and reduced pressure. Tape the edges where the new vent comes in contact with the unit face.
6. Cardboard is cheap and free, if this design does not meet your needs...do it again!