Rent or borrow a diamond-tipped cutting blade. You need a strong power tool to cut through the sandstone. If it is not sharp or fast enough, the stone could crack or you won't be able to penetrate it.
Draw chalk lines onto the sandstone slab where it should be cut. Use a ruler and a piece of chalk to mark these lines. Check at least twice to make sure you have done this correctly. If it is wrong, you can't put the slab back together. Remember to leave space for a joint gap. For example, if you measure 9.5 inches cut the slab at 9 inches to leave a 0.25 inch gap either side of the sandstone slab.
Place the slab on a steady surface for cutting. Firmly position it on a sandbag or something stable. If you don't have sandbag, you could lean it against the edge of a curb, a paver's maul or on a bed of bricks. This gives you cushioning between the ground and the slab, so that you do not cut the paving below or hit grass.
Assess the surrounding area where you plan to cut the sandstone slab. It will create a substantial amount of dust so get pets or bystanders out of the area. Make sure you are about 6.5 feet away from walls to give you enough room to maneuver.
Switch the blade on and begin cutting the sandstone. Start from one side and move to the other at a steady pace. Hold the blade with two hands for complete control. Cut halfway down the slab. Press down firmly and push the blade across the chalk line slowly.
Switch the blade off. Turn the sandstone slab over and then hammer the slab where you made the half-cut. The sandstone slab should snaps along the chalk line. Hit the slab with a strong, sharp blow from the hammer. If it does not crack, go back over the line with the blade again. This is a suitable technique for straight lines.