I cut a piece of scrap wood to be the "base" of the new window sill.
It was a tight fit so I nudged the wood into place with a rubber hammer.
Over the next two days I will be applying a insect/rot repellent and
a wood harder to stabilize the wood around and below the fix.
These 2 steps prevent any future rot around the repair.
I'll be back in a couple of days to
show you the progress.