remove the mounding around the window
Paint the side that will be facing outside black
I hang blinds in there , so from the outside it just looks like a dark room
Where you'll be venting through it, make sure to get HD blinds and anchor it to the bottom so it doesn't move around. Open the window b4 screwing the plywood.
Cut a hole for your exhaust , mount the board.
If your trying to be stealthy, and the room your using is somewhat sealed , blowing exhaust air out your window will create a negative pressure in the room and air will be rushing through any spot it can, making nose trying to equalize the pressure.
The more cfms the bigger this problem becomes.