The old sill is chiseled out or cut out entirely. A new预制 (pre-cut) stone sill is mortared into place.
Here is a full review of the techniques for repairing stone window sills, evaluating the pros, cons, and longevity of each approach. how to repair stone window sills
While the material is still "green" (partially set but not rock hard), use your trowel to mimic the profile of the original sill. If you are repairing a textured stone, you can lightly dab the repair with a dry sponge or a stiff brush to match the surrounding finish. 5. Finishing Touches The old sill is chiseled out or cut out entirely
(The gold standard for durability, but expensive). evaluating the pros
I can give you a specific or mixing ratio based on your answers.