I did two of these last December, but can only find one photo on my computer file. I had saved my Xmas cards from the previous year to do this after I read about it on a craft forum. I had an A3 art pad that I used for the background, glued old sheet music at random, then the cards over that.
The other collage was similar but I used cards with animals (cats, dogs, birds, etc) as the theme.