Sometimes whole concrete blocks will not fit into the space they're meant to be placed. You'll need to cut them to the proper size, and you have a few choices of how to go about making the break. The method you choose depends on how rough or how smooth you wish the cut to be and on what tools you have at your disposal.
Hammer and Chisel
Mark a layout line all the way around the concrete block using a straightedge to keep the line consistent and chalk to make the line visible.
Put on safety glasses, a dust mask and work gloves.
Score the block along all the layout lines by placing a chisel on the line and striking it with a hammer. Make the scored line about 1/8 inch deep.
Strike anywhere along the scored line using the chisel and a strong, swift hammer blow. The block will break in two with a rough, somewhat jagged cut along the scored line.
Choose a circular saw or a power masonry saw. Equip either saw with a masonry or diamond blade.
Don safety glasses, hearing protection and a dust mask.
Set the saw blade so it will cut about 1/4 inch to 1/2 inch deep.
Cut around the entire block with the saw, then adjust it so it will cut 1/4 inch to 1/2 inch deeper into the block. Make a series of passes until you have cut through the concrete block. A power saw produces a cleaner, more even cut than does chiseling.
Rent a concrete block splitter from a home center or tool rental store.
Place the concrete block on the platform beneath the cutting blade, and arrange the block so that the blade will slice it where you want the cut made.
Secure the block in place by spinning the tightening wheel on the splitter in a clockwise direction.
Pull the splitting lever downward, applying pressure on the lever until the concrete block splits. While this cut is precise, it is nearly as jagged as one made by scoring and splitting with a chisel.