Installing wood trim is one of the final jobs in finishing the interior of the home. Its job is to provide a smooth transition between walls, windows, and flooring. When cutting trim for 90 degree corners, you are required to cut each board with a 45 degree cut. This joins the two boards with one uniform joint. Cutting these boards is simple if you understand basic geometry. The proper tool for this job is an electric mitre saw.
Measure the wall from corner to corner where the trim is installed with a measuring tape
Transfer this measurement to the trim and mark the cut line with a carpenter's square and pencil.
Adjust your mitre saw to make a 45 degree cut. This is typically done with an adjustment mechanism on the back. Open the mechanism and angle the mitre saw until its gauge is aligned with the 45 degree marking and lock in place.
Align the trim's pencil line with the mitre saw's cut line and hold the trim in place with one hand. Make sure the hand holding the trim is sufficiently away from the blade during operation.
Activate the mitre saw with the other hand and guide the blade slowly into and through the wood.
Always wear eye protection when using a mitre saw.