The trim around your windows, both inside and outside the house, provides a visual break between the window casing and the surrounding wall, while also providing some additional protection against air flowing around the edges of the window. There are a few ways to cut and install the trim. One of the simplest for the do-it-yourselfer is a four-cornered mitre design.
Measure the width of the window opening along the top, from one upper corner to the other, using the tape measure
Transfer the measurement to a piece of trim, putting two pencil marks with the measured span between them. Make the marks on the front of the trim, along the bottom.
Lay the trim on your mitre saw, facing up, with the bottom edge lined up along the guide at the back of the platform. Move the piece so one of the two marks is directly in front of the blade. Swivel the blade to 45 degree outward, so it points away from the main span of the trim. Cut it.
Slide the trim so the second mark is in front of the blade. Swivel the blade to 45 degrees in the opposite direction as before (so it's again pointing away from the main span of the trim). Cut it.
Hold the trim along the top of the wind opening, so the two mitred corners rise up off the two top corners of the opening. Secure the piece with a trim nailer, shooting nails along the top and bottom edges every foot or so.
Follow the same procedure to cut the trim for the other three sides of the window opening. Nail them in place in the same manner. The angled cuts should form 90-degree corners.
Run unbroken beads of caulk around the perimeter of the trim, and around the inside of it.