Align the chop saw to make a 45 degree cut.

Consult the user's manual as needed to set up the saw correctly.

Align the trim on the saw so the mark you made is along the guide of the saw.

Again, the user's manual will provide specifics of how this is accomplished.

Operate the saw in a safe manner in accordance with the user's manual.

If you are uncomfortable using power tools, your local hardware store may cut the trim for you, using the measurements you made earlier.