Window sills can add beauty, architecture and value to your home. If your windows are bare, create a finished look by adding a window sill. The window sill refers to the bottom or base of the window opening. The sill is made up of two parts, the shelf is the base or bottom of the window and the apron is the decorative trim underneath the shelf.
Measure the window. First calculate the shelf, or bottom of the interior sill. As this acts a shelf, traditionally it extends out past the edge of the window on either side and 2.5 cm (1 inch) in the front. To the depth measurement, add 2.5 cm (1 inch). The width is the left to right measurement. The shelf will be inside the window opening so measure in a couple of places and use the smallest measurement.
Decide what type of wood to use for the window. The shelf should be a single piece of wood. Wood is available in different lengths and widths. You will want to purchase the size closest and easiest for you to manage that will need the least amount of cuts. If cost is a factor, also look into prefabricated wood product such as medium density fiberboard or MDF. MDF is affordable and can be used as a real wood substitute. What wood you choose will also depend on how you want to finish it. Use oak if you plan to stain. Choose poplar or another soft wood species for painting.
Choose the apron style. The apron is the decorative element under the shelf. The apron can be very ornate, or it can be a simple straight profile board. Choose the profile that will fit with the style of your home.
Cut the wood for the shelf according to the measurements. It can be cut straight, no mitred cuts are necessary. The shelf will extend on the sides and the front of the window opening 2.5 cm (1 inch). When you cut the wood, cut notches on the sides so that it sits within the frame. You will essentially cut a "T" shape into the shelf.
Cut the apron to line up with the window. While the shelf will extend beyond the opening, the apron will line up with the window opening.
Finish the wood either with paint or stain. The finish can be applied after installation, but it is actually easier to use paint or stain in an open, well-ventilated area on a flat surface. Use a brush or rag, depending on the manufacturer's directions on the product you have chosen.
After staining or painting the wood, apply two to three coats of a clear coat finish to seal the finish and add a protective coating that will resist scratches.
Wait an appropriate amount of time for the stain/paint to dry before continuing. Refer to the instructions on the stain/paint for specific instructions.
Install the shelf first. Pre-drill holes in the shelf to prevent the wood from splitting. Use finishing nails and the nail set to counter sink the screws. The screws should be slightly below the wood surface. Cover them with a wood putty that matches the finish you have applied to your shelf. You can use a liquid adhesive to the shelf, but as it is a flat surface, it is not always necessary.
Install the apron in the same manner as the shelf. Apply a liquid adhesive to the back of the apron before nailing it into place. Again, countersink the nail heads and apply wood putty to cover them.