How to Trim a Window Sill

Hold up a piece of casing next to each of the window's sides. Mark the position of the outside of the casing, or the side of the casing farthest from the window, on the wall. Measure across the window the distance between these two marks. Write this number down, then add 2 inches. This measurement will be for the stool, the trim around the window sill.

Set the miter saw to a 7-degree angle. Set the trim piece for the stool against the guide so it's standing on its side. Cut one end. Turn the piece around and make a mark at the measurement you calculated in Step 1. Cut this end.

Hold the stool in place. Place your compass in between the stool and the window, and open the compass until the point of the compass touches the window and the pencil touches the edge of the trim board.

Place the compass at the edge of the casing, with the point against the wall and the pencil on the face of the casing. Draw it forward along the casing, following the contour of any irregularities of the wall. The resulting line is where you will need to cut away the wood in order to fit the sill in place. Repeat with the other side of the window.

Cut along the lines you drew in Step 4 with a jig saw, then dry fit the sill in place against the window. If the fit is tight, continue to the next step. If there are gaps, retrim the board with your jig saw until the sill fits tightly.

Secure the trim with finish nails.