Repairing a rotten sill

If a small area of a wooden sill is rotten, but the rest of the window is sound, you can just replace this part of the sill

Tools and materials
Pencil, saw, router, wood, screws, drill-driver, plane

Cut back the sill to sound wood. Use it as a template for the patch. Mark the position of the groove under the sill (the drip groove).
Cut the patch slightly larger than the rotten section you removed. Rout out the drip groove. Screw the patch in place, and plane it smooth.

Other wooden frame repairs Another option for fixing loose joints is to strengthen them with dowel. If a whole section of your window is rotten, then prise the piece out and fashion a replacement, using the old section as a template. If rot is more widespread, you should replace the entire window.