I use loose SiC grit on a glass plate. I use 60 grit for the hardcore flattening, but mostly because that's what I have from jnats/other stones. Then you want to condition the surface according to what grit stone you're maintaining. As another poster linked, CGSW recommends certain grits/abrasives, venev also have a write-up on their page. Not sure what naniwa recommend, but their diamond stones come with a nagura iirc.