About 1/3 of the length of one of our basement window sill has almost fully rotted away (it is there in appearance but is as soft as paper).
The timbers are probably 100+ years and are dimensional.
What is the best way to fix? - Cut out the entire sill and replace with new wood - Cut out rotten part including a safety margin of good wood and replace with new piece (perhaps with scarf joint) being careful to make the transition as invisible as possible - Replace the rotten area with Bondo (though warning it is about a third of the entire sill)
Any other things to be careful about? Any good tutorial sources on the Internet?