An octagonal window can add a unique detail to a room, entry or hallway, and provide a beautiful glass element to your decor. Applying the interior and exterior trim is the final touch. Ensuring a professional finish is a matter of accurate measurements and cutting of the angles.
Measure 1/4-inch from the inside face of the window jamb and make a pencil mark beside the top angle on the face of the jamb. Repeat at the opposite end of the same jamb section. Use a straightedge to connect these two marks, extending the line past each corner. This will define the setback or "reveal" of the window trim.
Repeat Step 1 for each of the individual window jamb sections. This defines a continuous line, 1/4 inch from the inside edge of the jamb around the perimeter of the window jamb, and intersecting with the lines on the adjacent sections.
Measure the exact dimension between the crossing point of the lines for the first section of trim. This measurement is the "short" dimension between the two mitered cuts on the trim piece.
Set the miter saw to 22 1/2 degrees. Cut the end of a piece of window trim material, making sure the trim contour is positioned with its outside edge against the miter saw fence.
Measure the dimension determined in Step 3 from the short point of the angle cut in Step 4. Rotate the miter saw to 22 1/2 degrees on the opposite side of the saw's angle guide and cut the trim. This trim piece will have two opposing 22 1/2-degree angles at each end, and the Step 4 dimension at the short side.
Tack the trim piece cut in Step 5 into place on the window jamb with nails, making sure the inside corners match the intersections of the line marked on the jamb in Steps 1 and 2.
Repeat Steps 5 and 6 for the remaining seven pieces of window trim. Ensure each mitered angle matches the adjacent pieces of trim precisely and each joint is tight.
Secure the window trim pieces with additional 6d finish nails at the jamb edge and 8d finish nails for the outer edge. Drive nails approximately 8 inches apart, or as needed. Use a nail set to set all of the nail heads just below the surface. Fill the nail heads with putty before painting.