How to Create Old-Fashioned Window Sills

Measure the length and depth of the existing window sill.

Transfer the measurements to a piece of cardboard wider than the window. Draw the long straight edge that goes against the window and the opposite side that faces the room.

Measure any trim around the outer edges of the window frame. Transfer the measurements and shape of the outside edges of the window frame to the cardboard. An old-fashioned window sill usually extends 2 inches past the sides of the window, providing a decorative edge on each side.

Cut the cardboard to the size and shape of the window. Fit it carefully onto the existing sill, making sure it is an exact fit. If the window is a fixed pane, allow a 1/16 inch space between the glass and the cardboard. If the window opens, leave a space between the glass and cardboard to ensure that the window can open and close.

Tape the cardboard pattern to the top of the 1-by-6 board. Using a jigsaw, cut out the new window sill.

Fit the new window sill into the window frame. Attach by drilling from above, through the board and into the window frame.

Cut a piece of trim the length of the new window sill. Butt the trim up against the bottom of the sill, centering it under the window. Predrill and screw to the studs under the window.

Cover the screw heads with wood fill. Allow to dry completely and then sand the entire sill and trim with a 150-grit to 180-grit sandpaper.

Caulk around the sill and trim with a flexible silicon sealant. Smooth with a damp finger. Allow to dry completely.

Paint the sill and trim with a latex primer. Allow to dry and then brush on two coats of a semigloss or gloss latex enamel. Allow the paint to dry completely according to the manufacturer's directions.