Learn how to properly make miter joints, one of three joints used most often to install trim.
The miter joint, where the ends of the pieces to be joined are cut at an angle (usually 45 degrees), is one of three joints used most often to install trim. The others are the coped joint and the butt joint, where two square faces meet. Miter joints are used for outside corners, while coped joints handle most of the inside ones. Both allow molding profiles to continue around a corner without interruption.
To cut miters quickly and accurately, use a power miter saw, commonly called a chop saw, or a handsaw and a miter box. Most professionals use a chop saw for its speed and accuracy. A miter box can be just as accurate; however, it takes longer to make a cut by hand. If your budget is tight and you plan to do a limited amount of trimwork, opt for a miter box. You can buy a professional-quality integrated miter box and miter saw for the price of an entry-level chop saw.
Practice making miter cuts on scraps, both to improve your skills and to check the accuracy of your tools.
Place the test pieces in position. If the miter is open on the outside, adjust the saw to slightly less than 45 degrees. Make test cuts until the angle mates perfectly with one of the original test pieces. Cut one of the pieces you'll install to the adjusted angle, the other to exactly 45 degrees.
With corners that are slightly off 90 degrees, you can achieve a close fit for miter joints by cutting one molding piece to exactly 45 degrees and then making minor adjustments to the saw to "sneak up" on the matching angle for the other piece.
For corners that are intentionally far from square, you'll have to reset the saw to split the angle in half. Fortunately this is easy to do without math calculations. You just need a T-bevel and a compass.
Yet another method involves a protractor that accurately measures the angle.
Set a T-bevel against the corner to set the angle. Transfer the angle you want to a board.