The normal option is to buy a special flattening stone, but these can be quite costly. What works just as well, is to use 400 grit sandpaper, which you tape to a flat granite or marble surface.
I've criss-crossed the honing stone with pencil lines, and soaked the stone for 15 minutes in water. Then all I did was lightly rub both sides of my 3000/8000 grit stone on the sandpaper until the pencil marks were removed. That gave me nice flat surfaces to further hone my razor with.
My straight razor is now all honed and stropped, and ready for another two months or so of use before I need to hone it again.
See my tips at Traditional Wet Shaving | Gadgeteer
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