For guys using water stones here is a tutorial on how to flatten your stones that you might find useful.
How to flatten your hone That's a good tutorial. There is an easier and cheaper way to do which have been using for years on mine. I just find a flat section of concrete on the driveway or patio. Wet it down lay the stone face down apply pressure and work to stone across the concrete in either a "W" or circular motion. You will want to rotate the stone 180 degrees every dozen or so passes. I actually use both circular and "W" movements. The main thing is to keep the stone moving in a random pattern other wise you will end up removing more material from one section of the stone than others due to the uneven application of pressure. Rotating the stone while moving it in a "W" or circular pattern negates the unevenness.
The roughness of the concrete removes stone material fairly quickly however, during this process the stone is also polishing the concrete. If you have a lot of material to remove you will have to keep the stone on the rough. Once the stone is flat use the polished areas of the concrete to remove any scratches in the face of the stone. Don't forget the keep the concrete wet through the whole process