To cut the cement board to size, use a box cutter to score. I did this 5-10 times with firm pressure. Once you have cut through about half of the board, you can just knock it a few times with a hammer until it breaks off. To do this, I secured the “keep side” of my cement board to my workbench with clamps, leaving the “waste side” hanging off of the bench. Now, attach the cement board to the plywood top with multipurpose screws. (I used HardieBacker Cement Backerboard for this).