Lay the flagstone in the area where you will install it. Using a tape measure, measure how much stone must be cut off to obtain the desired shape. Mark the measurements onto both sides of the flagstone with a pencil.
Put on safety glasses and a dust mask. Lay a piece of plywood flat on the ground and on it set the flagstone that needs cut.
Line up the tip of the chisel with one end of the pencil line. Hit the top of the chisel two to three times with firm pressure using a hammer. This creates an indentation along the marked line on the flagstone.
Move the chisel tip to a new section of the pencil line and repeat the process of hitting it with the hammer. Continue moving the chisel tip until you have cut all the way down the pencil line.
Turn the flagstone over and repeat the process to cut all the way down the pencil line on this side of the stone. The flagstone will crack off along the chisel line as you near the end of the pencil line.