Improving energy for your home can be achieved by installing new windows. One of the most popular options is called a "stool and apron" style that is completed with mitered casing. Keep in mind that sometimes window trims do not fit perfectly the first time you try. To make a good fit, a small amount of drywall may need to be removed at the corner or a small amount gouged out from the backside of the casing. If that does not work, consider shaving just a tad off a miter, or the back edge can be planed. To ensure you get the trim right the first time, always measure twice before cutting the wood.
When cutting and installing window trim, you want to make sure you take your time. If you have both large and smaller windows to work on, start with the larger window. The reason this is important is that if you cut any of the wood too short, it can be used for the smaller windows. Always start by measuring the windows to determine the amount of molding required. If you do not own a power miter, this would be the ideal project to rent one, and always use caution. When using a power miter, place a fan on exhaust or reverse position to draw the dust out of the room if working inside. Additionally, you want to cover everything with a drop cloth as dust can get into even the smallest of spaces. If possible, work outside or in a garage or outdoor shed where the dust will not be as much of a problem and regardless of where you work never forget to wear safety goggles.
The window stool will need to be cut 1 �-inch longer that the outside dimension of the casing. While holding the casing in place, mark the outside edge on the wall and then do the same for the other side, measuring between the marks. Next, place a mark at the center of the angled sill and stool. Align the two marks and to fit the stool to the window, the notch width needs to be determined. Start by centering the stool on the window and then marking the edge at the inside for each side of the jamb. You may need to plane the window edge of the stool to ensure it aligns parallel to the sash. For the notch depth, measure from the stool to the sash, again at both jambs and then subtract 1/16-inch. Connect the two points, then mark and cut the notches. Bevel over the ends of the horns and using 8d finishing nails, tack the stool to the rough sill.
Now the jamb extensions need to be measured. If the jambs are not flush to the interior wall surface, they will need to be extended. Using a straightedge, hold it on the wall surface and measure from the jamb to the straightedge to determine the extension thickness needed. Look at both points on each side and on the top, choosing the largest of the dimensions. If you discover more than a 1/8-inch difference, just split the difference. If the jamb is wider than the wall, plane the jamb flush with the interior wall surface. For the jamb extension, strips of �-inch stock can be ripped to the correct thickness. Cut the head jamb extension full length and then glue and nail it, leaving 1/16 to 1/8-inch exposed. The side jamb extensions are then cut so they will fit securely between the head jamb extension and the stool.
The next step is to install the head casing by marking a 3/16-inch reveal on the head and side jambs. Measure between the marks on the side jambs and cut a 45-degree miter on both of the head casing ends to ensure the short dimension equals that distance. Now the casing needs to be positioned on the marks and tacked to the jamb using 4d finishing nails. The side casing is then measured from the stool to the top of the head casing for each side of the window. Miter cut each of the side casings to ensure the long point equals that dimension. Now apply glue to the miters, locking the joints together with a finishing nail from the top and sides. The side casings are then nailed to the jamb and wall.
For the return apron, make a mitered return on the ends and then cut an outside miter on the ends and on two short molding scraps. Glue and nail those scraps to the apron, forming a 90-degree corner at each of the ends. Once the glue is completely dry, cut off the excess with a utility knife and nail through the apron directly into the wall framing. Next, nail through the stool into the casing using 6d finishing nails. To finish the project, set the nails and fill the holes with wood filler. Sand smooth and then paint, stain, or varnish.