Beneath the window sill of many windows (particularly on double-hung windows), is a horizontal ledge, which extends just beyond the window casing at either end. This ledge juts into the interior of the room, and is known in carpenter-lingo as a stool. The window apron is a decorative finish that is attached to the window sill immediately below the stool.
Most window trims are made of either solid wood or fiberboard with a wood veneer. Maple, pine, oak, birch and poplar are the most common types of wood used for window trims. Plastic trims are equally as easy to cut as the wood trims, and sometimes easier. Pre-finish the trim by sanding, staining or painting it before you cut and install it.
Measure the length of the stool (which is usually equal in length to the outer casing line) and cut the apron to the same length. For square cuts, use a miter saw. To embellish the edges, use a coping saw. A coping saw is a relatively small, hand-held saw with a narrow blade. It has very small teeth to enable smooth cutting of tight curves. Coping saws are particularly handy for constructing turnbacks on your window aprons, as well as for cutting exactly along the beveled edges of moldings.
Using a coping saw, you can either cut on the pull stroke (with the teeth of the blade facing the handle) or alternatively, on the push stroke (with the teeth of the blade facing away from the handle). If the blade begins to diverge from the cutting line, withdraw the saw and cut from the opposite direction.
For a smooth finish, file down the edges of the window apron with sandpaper. Glue the edge of the apron to the window stool and nail it to the wall.