In this video, Ask This Old House general contractor Tom Silva shows a quick and accurate way to cut perfect-fit miters.
1. Make an angle-marking guide from two 16-inch-long 1x4s. Check to be sure that the two boards are exactly the same width.
2. Set one 1x4 against a wall; lay the other 1x4 on top and press it tight against the adjoining wall.
3. Draw a pencil along each edge of the top board to mark the bottom board.
4. Now use a pencil and steel rule to draw a line that connects the ends of the two previously drawn lines. This new line represents the bisected angle.
5. Stack the two 1x4s on top of each other and bring them over to the miter saw. Adjust the blade angle to match the bisected-angle line drawn across the 1x4.
6. Cut through both 1x4s, then hold them against the wall to check their fit. If necessary, readjust the blade angle and recut the 1x4s until they produce a tight-fitting joint.
7. Once you're satisfied with the miter angle, cut the molding to fit the wall.