Last time I was in a studio using natural light was about 25 years ago in college, where they had a really great set up. From what I can remember;
Ideally you want an east west room, so that you have skylights north and south. Skylights rather than windows give you better options - after all if you use standard height windows everything is lit from the side, whereas if the main light is elevated you can always bounce it around to fill in. The skylights need to have controllable blinds. Paint the room either matt black or white, but have sufficient reflectors of the opposite kind that you can control it as you like. For example if you have a white room you could install a curtain track aound the wall with black drapes. Obviously bear in mind that in most latitudes winter light, though beautiful is pretty dim, so natural light portraits do tend towards the static.