Take a look at this video on how he does it, as well as this selection of pictures from the Telegraph and the Guardian. There's another nice gallery here at plusmaths.org.
"Making these drawings is map-making in reverse.
You start with the map, and you need to make the ground agree with the map."
He has also been busy back home, taking to the sands off the Somerset coast armed with just a rake and a compass. There's more in BBC report.
For the latest news, including the chance to buy a copy of his new book, check in at his Facebook page (complete with nearly 300,000 likes) and website.