Measure the fireplace surround with a measuring tape. Use these measurements to mark where the cement board must be cut. Create these marks with a pencil.
Cut the cement board pieces to size. Place a straight edge along the areas of the cement board that must be cut. Run a utility knife along the straight edge to score the cement board. Finish cutting the cement board to size by snapping the board from the side that's not scored. The board should break cleanly when snapped.
Spread thinset mortar over the fireplace surround with a notched trowel. Press the cut pieces of cement board over the mortar.
Nail the cement board to the brick fireplace with 1½-inch concrete nails. Hammer the nails into the brick mortar joints where possible.
Allow the thinset mortar to dry overnight, then tile the fireplace as desired.