Most windows are built with a sill integrated into the frame but from time to time it may become necessary to replace old window sills with new or create a sill for a window that does not have one. Choose a wood grain that matches the existing wood for a window that has a stain finish, or choose a clear piece of pine or white wood for painted windows.
Measure the distance between the inside faces of the vertical window jambs. Measure the distance from the face of the trim below the window to the face of the window or sash frame at the bottom edge.
Mark this rectangle centred along one edge of a 3/4-inch thick piece of lumber a few inches wider than the distance from the face of the trim to the window frame.
Draw a line, using a straight edge along the edge of the rectangle and cut out the pieces on either side of it with a jigsaw.
Fit the rectangle of the new sill into the window frame and mark the outside edges of the trim on either side of the window onto the new sill. Use a pencil placed flat and upright against the side trim to mark the profile of any detail in the trim onto the new sill board.
Cut the sill board to length at the trim mark on either side with a mitre saw. Cut the profile of the trim carefully along the pencil line on either side. Sand the sill smooth by hand or with a power sander and 150 grit paper.
Remove any existing sill by tapping up from the bottom with a hammer. Use a flat pry bar to lift the old sill from the window if needed.
Tap the new sill into position in the window frame. Drive finish nails through the sill into the sub sill below every 10-to-12 inches.
Fill the nail holes with wood filler for stained frames or painter's caulk for painted frames. Apply a coat of stain or paint to match the window.