- 1). Measure the molding to determine its width. Measure the window and make a mark 1/4 inch from the inside of the window frame. Mark the window frame with the chalk all the way around the frame using the square, so that a box is drawn on the window 1/4 from the inside of the frame all the way around. Measure this box from side to side and top to bottom. To each of these measurements, add the width of the molding.
- 2). Cut the molding for the top of the window. Set the miter box for a 45 degree cut. Place one end of the molding in the miter box, and make the cut on that end. The side with the longer end of the cut is the outside of the molding. Determine the size needed, the width of the chalk box plus the width of the molding. Measure down the outside of the molding from the tip to the length needed, and make a chalk mark. Place this end of the molding into the miter box so that the chalk mark will be on the corner of the outside edge of the cut, and make the cut.
- 3). Cut the bottom piece of molding with the same measurements and using the same process as for the top molding.
- 4). Cut both side moldings. These moldings are cut and measured in the same way as the top and bottom moldings, but the measurements for both side pieces along the outside edges is the length of the chalk box from top to bottom.
- 5). Install the top molding. Line up the inside edge of the top molding with the top of the chalk box. This should leave a small lip of the window frame showing, called the reveal. Use finishing nails every 16 to 18 inches to attach the molding, driving a nail at each place into the window frame and another into the wall framing around the window.
- 6). Install the two side pieces. Line the miter cuts up with the miter cuts on the top molding. The corners of both pieces should match up with the corners of the top, and the inside edges of the side pieces should line up with the two sides of the chalk box. Attach with finishing nails in both the window frame and wall frame every 16 to 18 inches.
- 7). Install the bottom molding. The miter cuts should line up with the miter cuts of the side moldings and the inside edge should line up the chalk box. Attach with finishing nails in the same way as the other pieces of molding.
- 8). Wipe any remaining chalk away from the reveal and ensure the molding is attached firmly.