Designing Kitchens and Bathrooms - Tiling Shower Window Sill

I broke the tile on our window sill in the shower. I'm thinking I'll take out all of the tiles on the window sill and put in one large piece or corian or something similar that is tilted downward so the water rolls off. We had a new window installed last fall, so taking out the window is not an option during this process. My question is, how to I install a new chunk of material on the window sill, at an angle? Do I treat this installation like a floor tile job? I am unsure as to what materials I need and how to do it...

I would clean it up and use a good modified thinset to set the new tile. Add just a little more thinset in the back of the tile and set it creating a slope so the water runs away from the window. Caulk the the edges and your done.