Cut out rotten bit of windowsillSee, I've learnt something right there, tell me more about this.^.
Make sure you leave a rough surface to join to and mix up some body filler
Fill gap or remake corner etc to rough shape
After 20 minutes you can file and sand a lot easier than wood
Then repaint windowsill
Will outlast the rest of the sill
If remaking a corner or end screw a couple of brass screws in so filler has something good to grip on