Re: replacing inside window sills
It's not difficult at all, particularly if you can salvage the existing sill as a template for the new one.
Cut around the edges with a utility knife to sever any caulked joints. With a flat bar carefully pry off the apron (strip under the sill on the wall face ), then work the bar under the sill. Work the sill loose all the way along it's length a little at a time so as not to break it (use later as template ). Once it's off, pull any nails left behind and repair any damage made to the wall.
Using the old sill as a template, trace it's pattern onto the new material. I strongly suggest using wood and staying away from MDF and other composite materials that will swell if they get wet, which is common against windows that sweat. Drop the new sill into place and place two nails on each end as close to the edges as possible and still have good purchase. Depending on the length of the sill you'll need to place nails along the center as well. Cut and install a new apron. Done!
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!