Lighting – How to Cover Small Window but allow Light

Start by removing the trim from around the window, this will give you a good look at the problem. Once you have the trim removed you'll be looking at something similar to this...

You may notice gaps between the window frame and the wood frame or between the wood frame and the wall, these are the areas you'll want to fill to stop the draft.

Get a can spray foam (like GreatStuff they make some specifically for this purpose, which is "low pressure" so it will not bow the window frame), fill any gaps (be careful not to use too much, this stuff can expand quite a bit). Once the foam has hardened (about an hour), use a sharp knife to trim the excess foam so it's flush with the window frame. Reinstall the window trim, and your done.

Buy paper window shades. Specifically the ones that are folded like an accordion and have an adhesive edge at the top. Trim them to the width of your windows. Lay them out flat and paint one or both sides black with spray paint. Stick them up behind the Venetian blinds. If you only painted one side then you can face the white side out if you are concerned about the external appearance.

During the day you can lift them up and clip them in place with those plastic clips that are used to keep chip bags closed. Or if the windows are double-hung you can accordion them up and sit them on the top of the bottom sash.