Chinese white watercolor in a tube or block works great and simply, once you know the technique.
For the block, wet your finger (I wouldn't lick it since I don't know what's in the paint) and rub it on the block of paint, then smear it on the work piece as Lucky said previously. You just want to dull the surface, not "paint" it. This will dry in a minute or two and you can draw on it with a 0.5mm mechanical pencil with ease. To erase use a wet finger and start over.
If you have the tube type paint, simply put a small dab on a wet paper towel sitting in a small dish or jar lid. Again, wet your finger and dab it in the paint and proceed as before.
Jim Chambers can tell you about a more high-tech method with computers,, scanners, printers and transparencies, that can yield much better detail, but he's still at Friendship at the moment.