You can use baking paper either stuck on to the window frame or hang easy to make blinds where wooden dowelling is attached to a roll of baking paper (cut to size) at the top and bottom.
Some have used shower curtains and sheets. Check how thick they are before the actual night in case you lose image definition.
Windows with multiple glass panes
Some houses have one big window which is perfect for showing a projected image. Others however have windows made up of a number of panes of glass: 2, 4 and sometimes more. For an image this might not be a problem but again check before the night. The eye has a way of seeing the individual parts as a whole image. If what you are projecting includes words then wooden struts dividing panes can make it difficult to read them. For that reason I try and avoid words whenever possible.
You don’t have to use the whole window of course and instead project on to a single pane of glass. One illuminator in the past has used a picture frame against the window and projected into that.
In many cases you will be back projecting ie you will project the image on to one side of the screen and the viewer will see it from the other side.
This means that unless you change the settings then the image to the viewer will be back to front. For pictures this might not be a problem. If though what you are showing includes words then it will be.
You can always flip the image so that it will look the right way round to the viewer. You can do this either by using the settings on the device (computer / tablet) that you’ve connected to the projector or some projectors have an easy way of flipping images too.
You might not have been expecting this one but from experience we know that sometimes what looks like a crisp image at the start of the evening begins to blur as condensation builds up on the window. The solution is easy. In fact it is soapy. It works on exactly the same principle as stopping your bathroom mirror steaming up. Wash the inside of the window with soapy water. This will last for a few days and stop condensation forming and your beautiful image disappearing behind a veil of mist.
Looping films and avoiding blackouts
If you are connecting a computer or tablet to a computer and showing a film then to avoid you having to start it every time its ends then in the settings put it on a loop.
Another thing to remember is that most computers and tablets will have an energy saving arrangement so that after a period of time it will enter sleep mode and the screen will shutdown. Check before the night and if necessary change the settings.
If you plan to include sound then a way which doesn’t involve you turning the volume up to full blast, opening the window and disturbing your neighbours is to use bluetooth or an equivalent for your computer and placing a full charged speaker device on a top windowsill in a plastic bag in case of rain.
Connecting projectors to computers etc
This depends on what type of projector you are using. My advice is try it out beforehand. If you are borrowing it from work or someone you know then ask them how you connect it up. Failing that you can use a search engine on line.