How do you flatten a sharpening stone?

How do you flatten a sharpening stone?

Start with a reliably flat surface, such as a piece of 1/2″ thick glass (with edges ground smooth).

Apply 100-grit, self-adhesive, wet-dry sandpaper and rub the wetted (or oiled) stone over it until flat.

Repeat with a few strokes on 400-grit paper to remove any course scratches.