Use the broken one as a template...same thickness of wood, same shape and size. Cut from new lumber, stain to match the other woodwork or paint. Note where screws or nails had been. It may take a bit of tweaking...dry fit the pieces before anchoring in place.
Whenever I think of window sills, I always think of them being inside the house so my cat can sit looking out the window. Am I old fashioned? Do they not put window sills inside houses anymore?
Use the broken one as a template...same thickness of wood, same shape and size. Cut from new lumber, stain to match the other woodwork or paint. Note where screws or nails had been. It may take a bit of tweaking...dry fit the pieces before anchoring in place.
This video will explain all the parts of the sill and how to replace it.
https://www.youtube.com/watch?v=x8iu9dLrV-E