Before I built my darkroom, I used to tape up window with the black plastic sheet you get at hardware stores (I got mine at Ace, but should be quite common). tape it up with masking tape (the blue paint masking tape is not strong enough, I used a cream colored one, I don't seem to remember what it is called). You will need to make 2-3 layers of the plastic sheet and tape up on all sides.
See "Build Your Own Home Darkroom" by Lista Duren, Will McDonald for ideas about making a vent using fans(they talk about exhaust fans - just reverse the fan and you should be all set).