BBC Four did an Instructable: How to make a camera obscura
For some reason the video-embed says the video is no longer available. It is, I just watched it.
The link is http://www.youtube.com/watch?v=Xcv4CxQ1I-A
But, if that doesn't work either:
An Italian photographer takes over a room in a Venetian palace. They cover all the windows in opaque black plastic sheets. They wait a moment to let their eyes get used to the dark, then cut a hole in the plastic, about 3 inches square.
An image of the view outside appears "magically" on the wall opposite - upside down, but perfect in every colourful detail.