Windows Without Internal Sills . You can now use the crowbar to remove the window sill. Window sills are important to the design and functionality of the window. How to install a window sill and trim Sawdust Girl® from sawdustgirl.com Drill a 3/8 hole in the new block, and the old sill in the same spot. Cut the trim long so it butts up to the apron. At the back there will be a big picture window (grey frame) and the builder intended to put a traditional white sill below the frame, saying that it’s “normal” for the sills, architraves and skirting boards to match.