This window sill was broken and the piece that broke off was lost.
Make a template out of cardboard by tracing and then cutting out the shape of the missing piece.
Trace the template onto a piece of wood of the appropriate thickness and cut it out as close as you can. Before you cut, check to see if the edge broke at an angle. It usually does and it helps to try to replicate this angle in the "patch" you are cutting out. Check the patch for fit and, if you like what you see, use a router to shape the front of the patch to match the trim, if applicable.
Nail on the patch and fill any gaps with wood
Sand the wood filler.
Then, prime and paint or, if you must, stain and varnish. Repairing trim works best with painted trim, however, since the variation in grain and any wood filler will not be visible.