Trimming a Window Using Corner Blocks

Check to ensure the window jamb is flush with the surface of the wall by using a carpenter's square or straightedge. Plane the jamb with a surface plane if it isn’t flush.

Nail or glue thin strips of window molding over top of the jamb to make it flush if it falls short of the wall surface. Plane away any extra after nailing or gluing it on.
Mark the reveal lines around the area of the window with the straightedge of your carpenter's square. Set the casing back about 1/8 inch from the jamb unless you’re using a different distance for design purposes.

Position a side casing on the wall and align it with the reveal line on one of the side jambs. Make a mark on the wall at the outside edge of the casing. Repeat on the other jamb.

Measure the distance between the two marks and add 2 inches. Cut the window stool to this length and place it on the window sill face down with the back edge on the window sill trim. Mark where the end of the stool and top of the trim meet.

Hold the stool upside down and place the end of it against the jamb. Draw a vertical mark where the jamb touches the end of the stool. Make a square to connect the marks and draw an "X" in the area defined by the lines. Make two different cuts on a table saw to remove the waste.

Mark the inside edge of the stool 1 inch from each end and position it so it’s across the opening. Mark where the edge of the stool and the inside edge of the jambs meet.

Position a square at those marks and draw lines across the top surface of the stool. Use a router to make your design in the ends and edge of the stool.

Cut a notch into each end of the stool with a jigsaw. Test fit the stool in place and detail the fit with a chisel, jigsaw or sandpaper until it is just right. Lay a bead of glue along the window sill and use four 18-gauge nails to face nail the stool to the window sill.

Cut a straight edge on a piece of side casing and stand it up on the stool. Make a square cut at the point where the inside edge of the side casing meets the reveal line of the head jamb. Repeat on the other side.

Line up the edge of the side casing to the reveal line after adding a bead of glue where it overlaps the side jamb and where it sits on the stool. Face nail the casing on the jamb with finishing nails.

Position a piece of head casing along the top of the window and make a mark where it meets the inside edge of each of the side casings. Cut square crosscuts at the marks with a miter saw and glue the casing to the head jamb so the bottom edge and reveal line are lined up.

Hold your corner blocks in pace in the corners of the window frame to test whether they fit properly. Plane the edges down as needed until they fit securely.

Lay a thin line of glue along the edges of the corner blocks that will touch the head and side casing. Place glue on the back and secure each corner block in place. Drive a couple of finishing nails into each corner block.