Put on rubber gloves and safety glasses. Spray stone cleaner all over the windowsill if you purchased it in a spray bottle, or pour it in a bucket and mix with water according to the directions if you purchased a concentrated form. After mixing it, apply it to the sill with a soft sponge.
Scrub the limestone windowsill until the sponge is dirty. Rinse the sponge in clean water and apply more stone cleaner to the windowsill. Scrub it again with the sponge, and repeat the process until no more soil transfers to the sponge.
Rinse out the sponge. Wipe the windowsill with the rinsed sponge to remove leftover cleaner.
Buff the limestone windowsill with a soft towel to dry it.