Easiest/cheapest way to effectively flatten stones?

[knives_in_nature]

I have some edge pro stones that I have sharpened maybe ten knives with and I need to flatten them. Is there a cheap and easy way with sandpaper or something? Or should I just order a specific carbide or flattening stone.

 

[Hytekrednek]

I think the cheapest way is with sic carbide grit. It is very fast and effective. The grit is cheap and a piece of glass to use the grit with is easy and cheap to get. Wet/Dry sandpaper on glass works too, but is slower and the paper does not last long in my experience.