How to Replace an Interior Window Sill With Faux Marble

Run a utility knife around the perimeter of the old window sill, slicing the caulk sealing the sill to the window frame and the drywall or tile below.

Force a pry bar into the seam below the old window sill. Lift the front edge of the sill off the window opening's sill plate, using the pry bar for leverage. The window sill bonds against the building's sill plate.

Pull the old window sill out of the window opening.

Measure the length of the sill plate with a tape measure. Subtract 1/4 inch from the measurement and transfer the total to the faux marble window sill's finished surface, using a pencil to place a measurement mark at the appropriate spot.

Draw a straight pencil line across the faux marble window sill's finished surface at the measurement mark, using a carpenter's square as a straightedge.

Cut the faux marble window sill across the pencil line with an angle grinder equipped with a tile-cutting blade. Run the blade back and forth across the faux marble until the blade reaches the unfinished side. Trim the sharp edge off the unfinished edge with the side of the tile-cutting blade, running the blade across the sharp edge with circular motions.

Run a 1/4-inch-wide S-shaped bead of colored silicone caulk across the sill plate. Use a colored caulk that matches the faux marble's color.

Set the faux marble window sill onto the sill plate. Center the window sill in the window's opening, leaving a 1/8-inch-wide expansion gap on each side of the window sill. Press the sill down onto the silicone caulking.

Set a bubble level on the faux marble window sill's surface. Adjust the window sill for plumb, using the bubble level as a guide.

Apply a 1/4-inch-wide bead of the colored silicone caulking around the perimeter of the faux marble window sill. Clean the excess caulking from the window sill with a rag.