How to Trim Out Window Sills

Measure across the top of the window opening from corner to corner.

Mark out the measurement on a piece of trim, with two marks and the measured span between them.

Lay the trim on your miter saw, facing up, so the bottom side is running along the back rail of the saw and one of the marks is under the blade. Swivel the blade 45 degrees outward (toward the close end of the trim). Make the cut.

Move the trim so the other mark is under the blade. Turn the blade to 45 degrees in the other direction. Make the cut.

Set the piece of trim along the upper edge of the window opening, so the bottom of the trim is right at the edge and the two miter cuts angle up and out from the two corners. Secure it with your hammer or nail gun, shooting nails along the top and bottom edges of the piece every 10 to 12 inches.

Measure one vertical side of the opening, from the inside corner on top to the horizontal top of the sill at the bottom. Transfer the measurement to a piece of trim.

Cut the trim's upper end as before, at a 45-degree angle outward. Cut the bottom end straight across. Nail the trim in with the angled top end butted to the miter of the upper piece to form the corner. Line up the straight-cut bottom end with the top of the sill.

Repeat Step 7 for the other vertical side of the window opening.

Measure the width of the window plus the surrounding vertical trim. Cut a fourth piece of trim to this width, with both ends at a straight 90-degree angle. This piece will be about half an inch narrower than the sill and is called the "apron."

Nail the apron under the sill, centered, with the upper long edge of the trim butted under the width of the sill.

Caulk all around the window's new trim wherever it meets the wall, the window casing and the other trim.