Trimming a window with a stool and apron is more complicated than trimming a picture frame window, even though there are few, if any, miter joints. If the head and side casings have the same dimensions, they can be mitered at the top. If they have different dimensions, they must be butted.
To trim a window with this style of casing, start with the stool. Stool stock is available in a variety of profiles; choose one that matches the windowsill. The most common stools assume a sloped windowsill. For flat windowsills, the stool can be 1-by material (the front edge can be rounded) that is cut to fit and simply butted against the sill or the window frame.
Begin by cutting the stool to the finished length. This length is equal to the inside distance between the jambs plus the lengths of the two horns. Each horn is equal to the width of the side casing, plus the reveal, plus the amount the stool extends beyond the side casing.
Next, mark the centerlines of the sill and stool and hold the stool against the window opening. If the window sash (or finished sill) is not parallel to the edge of the stool, scribe a cutting line along the stool with a compass. Cut along this line. Then, holding the stool with the centerlines aligned, position a combination square on the stool and push its blade tight to the finished jamb. Draw a light pencil line across the stool.
Determine the depth of each horn by setting a compass to the distance between the windowsill or window sash and the edge of the stool. Use this setting to scribe the stool parallel to the wall at the horns. This is the cut line; it ends at the point where it intersects with the cut line of the jamb (the line scribed against the combination square).
Make these cuts with a jigsaw or a handsaw. If the jambs are not square, leave a little extra material at the horns as you cut; then plane or rasp it to fit. Place the stool in position and check to see that it is level and square. If necessary, insert shims under the stool to bring it up to the proper level relative to the windowsill or sash. (The stool should be about 3/16 inch higher than the windowsill or the sash). Use the appropriate-sized finishing nails to fasten the stool, driving them through the shims at 8 to 10 inches on center. On long horns, nail horizontally through the horn into the trimmer.
Make the first casing by cutting a piece of stock square at one end and a little longer than the final length. Position the squared end on the stool, aligning the edge of the piece with the reveal marks. If it doesn’t fit, plane or rasp as necessary. When the fit is satisfactory, mark a cut line on the piece at the head casing reveal mark. Make a square cut at this line, position the piece, and tack it to the jamb and trimmer. Repeat for the other side casing.
If the side casings are to be mitered at the top, proceed as described above but at the reveal mark, cut a 45-degree miter instead of a square cut. Then tack the casing in place.
Mark the head casing by measuring the distance between the outside edges of the side casings and adding twice the amount of the head casing overhang. This dimension should equal the length of the stool. Position the head casing on top of the side casings and equalize the overhangs. If the butt joints are not tight, remove the side casings and adjust as necessary for fit and reveal. Fasten the casings to the trimmer and to the jamb with appropriate-sized finishing nails, spaced at about 12 inches on center. A mitered head casing is installed like a picture casing.
The material that you used for the side casings, or something similar, can be used for the apron, but the apron should not be wider than the casings, for aesthetic reasons. Measure the side casings, outside to outside, and cut the apron to that length. If the exposed end grain is unsuitable, it can be shaped with a router or a coping saw, or you can mount a return on the end of the apron to give it finished appearance. Position the apron and drive the appropriate-sized finishing nails into the rough sill at about 12 inches on center. Then prop a temporary support under the apron and attach it by driving 6d finishing nails down through the stool the top of the apron.