Remove the excess bracket. Make sure to remove parts of the bracket that do not contain holes. These holes are used for fasteners.

To cut a section, make sure you make perfectly vertical cuts for the remaining pieces to line up. To assure a straight line to cut, use a sharpie marker to mark a straight line.

Use a reciprocating power saw or a grinder with cutting wheel. I used a grinder because I found it to be easier.

Maintain eye protection, and gloves if necessary.