Cutting a 45-degree angle through a material requires first marking the material in the correct location and at the right angle. The material can then be sawn through on the marked line using a circular saw or powered mitre saw. The benefit of the mitre saw is that once the saw base is set to the 45-degree mitre cut and the material correctly placed on the saw bed, the cut is virtually guaranteed to be sawn straight.
Measure and mark in the correct position one edge of the flat side of the material, using a tape measure and carpenter's pencil. Rest a carpenter's 45-degree set square against the flat edge with the mark, so that the square's 45-degree edge faces the desired way to mark the cut -- make sure the end of the 45-degree edge rests over the pencil mark. Run the carpenter's pencil tip along the 45-degree edge of the set square, marking along the material surface. Remove the set square.
Rest the bed of a circular saw on the material's surface with its vertical saw blade 1/2 inch from the beginning of the marked 45-degree line. Also, position the saw blade to follow and cut along the marked line. Pull the saw's trigger and slowly push the moving saw blade along the 45-degree pencil line, making sure it does not deviate to the left or right of the line. Follow the saw through to the end of the line to cut through the material.
Turn the locking handle on the mitre saw bed counter-clockwise (if choosing to cut with a mitre saw), and move it to the 45-degree setting on either the left or right side of the central 90-degree setting -- which 45 degree setting you use depends on which way the 45-degree cutting line was marked on the material's surface. Turn the locking handle clockwise to lock and secure the 45-degree cut setting.
Rest the material on the mitre saw bed and against its vertical back fence. Position the marked 4- degree line on the material directly under the vertical saw blade. Hold the material in position with one hand (6 inches or more away from the saw blade), squeeze the trigger on the saw arm and lower the saw arm/blade down through the material until the material is cut through. Raise the saw arm back to its original position and remove the cut material from the mitre saw bed.
Keep both hands away from the moving mitre saw blade when cutting through the material.