More than 800 years ago Italian paper makers started using watermarks to differentiate between paper made by different craftsmen. Each artist created their own watermark, which was unique to their work.
They made their mark while the paper was still wet. A dandy roll, a wire-gauze cylinder marked with their design, was used to mould the paper.
Once dry the mark was only visible when held up to the light. It wasn't until later that the watermark was used for security purposes, marking banknotes to avoid forgeries.
If you make your own paper you can mark your design once the pulp has been fashioned into a flat shape, but before it is dry.
However, you can also mark paper you have not made. Take a piece of blank paper and soak it in a tray of water until it is soggy, but don't leave it too long as you need to remove it in one piece. Place your sheet of wet paper on a mirror, and use a pen with its lid on to mark your design on the paper. Do not push too hard or you will cause too much damage to the paper.
Leave the paper to dry. When it is dry it should look like an ordinary piece of paper, but when you hold it up to the light you should see your watermark.
By experimenting with different designs you will soon be able to formulate some guidelines, such as keeping your design simple, and not working too close to the edge of the paper. You could also explore other ways of making your mark, perhaps using household items such as stamps, or making your own dandy roll.
Sophie Duncan is project manager for science at the BBC www.bbc.co.ukscience