You can install an inexpensive bathroom exhaust and duct it outside, also install a wall vent(or two) that is light tight(as per Ralph L.). I would put one intake vent near the ceiling, and another near the floor, and a power exhaust in the ceiling. Make sure to place the 'in' at the opposite end of the room from the 'out'. A weather seal around the door will help to shut out light. When you are finished, sit in the darkened room for at least five minutes before checking for light leaks.