Determine the size of the block that you'll need for your job. In most cases, you'll be adding a portion of block to the top of a column to complete the support. Subtract the height of the existing column from the planned height to determine the block size.
Apply the measurement to the block with a ruler and pencil. Use a straight edge to draw a line across the block on both sides.
Put on work gloves and safety glasses. Place the tip of the chisel on the line at one edge of the block. Tap the chisel with a masonry hammer to lightly score the block. Shift the chisel over and tap again to continue the scoring. Continue until the block is scored all the way across the line.
Turn the block over and repeat the scoring on the opposite side.
Firmly tap the portion of the block to be removed with the masonry hammer until the undesired portion breaks off. Remove any remaining projections by placing the tip of the chisel against them and tapping the chisel with the masonry hammer.