Mark the end cut measurements directly on the cement fiberboard with a tape measure and a pencil. For example, if the cut requires you to remove 1-1/2 inches from one end of the board and 2-3/4-inches from the other end of the board, use a pencil mark to mark the measurements.
Lay a straight edge on top of the cement fiberboard, and line it up with the pencil marks. Draw a straight line on the board from one mark to the other to use as a cutting guideline.
Load a 7-inch masonry blade into a 7-1/4-inch circular saw. Install the blade so it will spin in the direction specified by the manufacturer. The direction is specified on the masonry blade with a directional arrow. Be sure the retaining nut is tight.
Put on a dust mask, safety glasses and hearing protection.
Line the saw blade up with the pencil guideline. Turn the saw on and feed the cement fiberboard into the saw slowly so the blade does not bind. Allow the saw to complete the cut as quickly as it is capable. Hold the trimmed piece of cement fiberboard with one hand as you complete the cut so the trimmed piece does no fall freely to the ground.