There are many ways to flatten a sharpening stone. What is most important, though, is that you are working with a perfectly flat surface.
For instance, you can take a piece of 600-800 Grit Wet/Dry sandpaper, and tape it (with the abrasive side facing up) to a marble or stone kitchen counter, or any sturdy glass countertop; spray the paper with water, and you are all set! (if you are using this method, a helpful tip is to draw a graph with a pencil on the surface of the stone you are lapping, and lap until it no longer shows)
Diamond plates (specifically DMT steel plates) are another great option, and although a little pricey, they are incredibly effective. Not only are they already perfectly flat, but stay that way.
Stone Lapping plates are usually flawed because they NEVER come truly flat. So why are we selling this one? As you'll find, Naniwa is the exception, as this stone is not only ultra hard, but also completely flat. Keep in mind it is 220 Grit, and works fast. This means you will have to smooth it out after with 800 Grit sandpaper, to remove any of the 220 Grit grain that is left behind after each use; or, of course, use a rub/slurry stone. This lapping plate can work with any stone on the market, it's that BIG.