It's a secret, arcane art, like witchcraft or alchemy. All mouthpiece guys learn through trial-and-error or as apprentices, like sorcerers. If somebody has put a flat curve on your mouthpieces that won't play, you're in deep doo-doo. The table will have to be taken down (if possible) to generate material to then cut down to create a new facing. Depending on how bad it is, it may not be possible to fix them.