Mark on the door where you want the square on the panel, using a square and pencil. Make sure the outer edges of the marked area don't interfere with the knobs or hinges.
Measure the upper span of the square and mark that measurement on a piece of trim, with two marks along the top and the measured span between them.
Lay the trim on a miter saw, with the upper edge of the trim along the blade guard. Move the piece so one mark is under the blade. Turn the blade 45 degrees in, so it points to the middle of the trim. Make the cut.
Move the trim to put the second mark in front of the blade. Turn the blade 45 degrees so it's pointing in the opposite direction as before. Make the cut.
Apply carpenter's glue over the back of the cut trim piece. Set it to the door, with the top edge of the trim aligned along the top edge of the marked square. The mitered ends should angle in and under the two top corners.
Repeat the process for each side of the marked square. The mitered ends of the pieces should butt tightly at the corners to make the square. Do each door in the cabinet in the same manner. Let them dry overnight before installing the doors.