I love old windows! I like to find unusual ways to repurpose them. I watched Durwin Rice's video, you should check it out here. I decoupaged pieces from an old calendar.
I cut out the pieces of the calendar, and watered down some regular Elmer's glue-all. I coated the front of the piece then laid it on the window. I then spread more over the back of the piece and used my fingers to rub and massage it to get out all of the air bubbles.
This huge calendar was perfect for the large window panes.
In this photo, you can see through the window to the wall. I was trying to decide if I should leave it plain, or put mat board behind it.
Here it shows a neutral mat board with a slight texture.
This is actually the back of the mat board. (white)