Typically, to apply a poultice, you stir a liquid — sometimes just water — into a powder, such as cornstarch, baby powder, baking soda or one of the many poultice preparations made for stone care. When the mixture has enough liquid to reach a paste consistency, you spread it like peanut butter over the stain and wait. The liquid sinks into the stone and dissolves the stain. Then the liquid moves to the surface as it evaporates, pulling the stain ingredients along for the ride. When the paste finally dries, the crust contains a significant amount of what was causing the stain. After wiping or vacuuming that away, if you still see stains, you can apply one or more additional poultices. With a fresh stain, you might be able to remove nearly all the color. With an older stain, some of it might be permanent.
https://www.washingtonpost.com/home/2023/10/20/advice-remove-stains-marble-stone/