Mark the place you want to cut on your paving slab. Use whatever is handy--a pencil or chalk, for instance.
Use a chisel and mallet to cut the paving slab. Score along the line you drew by simply hitting the chisel firmly with the mallet. Do not try to cut the paving slab with one or two hits of the mallet on the chisel. It is better to work your way across the length of the paving slab and then go back to the beginning to start another pass, going deeper into the slab each time until it snaps in two.
Use a guillotine paving stone splitter to cut the slab. Place the paving slab into the slot on the machine with the line where the blade will come down onto it. Bring the level down firmly onto the slab. Repeat this several times until the slab snaps in two.
Use a diamond-tipped blade masonry saw or concrete saw to cut the paving slab. This works just like a hand-held power saw that you would use to cut wood. The diamond-tipped blade, however, will cut through the stone, whereas a blade for a regular wood saw will not. Run some water along the area you wish to cut while running the saw. This will keep the saw blade from burning up and make it easier to make your cut.