How to Cut Corner Molding Around Windows

Use the measuring tape and find the overall width measurement above the top piece of window trim.

Add 1/16 of an inch to this measurement. This will help take into effect the cutting kerf from the miter saw blade. The kerf of a saw blade is the amount of material removed by the blade or equal to the width of the cutting blade.

Use the miter saw and adjust it to a horizontal 45-degree angle cut. Cut one end of the corner trim at the 45-degree angle.

Place the end of the measuring tape at the long end of the 45-degree angle and mark the other end of the corner molding. Use the measurement you found in Step 2.

Cut this mark at a 45-degree angle just as the other end was cut. One side of the corner molding should be longer, the outside of the trim. And the other side shorter, the inside of the trim.

Attach the corner molding to the top of the widow trim with the brad nailer. Use your fingers to get a feel for the ends of the molding with the edge of the window trim. You should have an even amount of overhang on both sides, less than 1/32 of an inch.

Measure the bottom side of the window trim and perform the same steps described above. Do not forget to add 1/16 of an inch to the overall length for the bottom corner molding.

Attach the bottom corner molding as you did the top piece.

Measure between the top and bottom pieces of corner molding for the side pieces. You can measure either the inside or outside of the 45-degree angled cut. Be sure that the tape measure runs parallel to the window trim. Do not measure the length at an angle such as inside to outside of the mitered cut. This measurement must be exact so the joint between the corners will be a tight fit.

Cut one of the side pieces of trim and test the fit before brad-nailing it into place.

Cut the second side piece and test for fit. If both pieces have a good tight joint, brad-nail into place. You can now move on to the other windows.