Position the reflector on the opposite side of the cake from the window (having the reflector sitting on a chair or held by assistant is best).
Angle the board so as to reduce your shadows slightly, playing around with it to see what you prefer.
Round, tiered, or sculpted cakes take a bit of careful reflector work. If you have too much light from the one side, you lose the other in shadow. If you balance it out evenly, you lose the dimensionality. Make sure your reflector is close enough to cancel the shadows on the dark side about halfway. From there, fiddle around with it until you get it where you want.