Haha, not stupid, I just learnt how to do it myself an hour ago! For it to work though, your image must already have a dark background, evenly lit and close to black.
* Use Noise Ninja to remove ISO noise from the photo (may not be applicable to your photo)
* Open photo in Photoshop and crop/rotate
* Open up levels (Ctrl+L)
* Drag the 'black point' (the little black triangle on the bottom left of the histogram)
* You'll find your photo may now be lacking a little colour, so increase the saturation, and play with the levels some more (but choose a specific color channel in the dropdown on the levels tool).
As for my lightbox, the top is just some tissue paper I found in the cupboard.
Tom.