Measure across the top of your stone slab and subtract 2 feet from the measurement. For instance, if your stone slab is 3 feet long, your final measurement will be 1 foot.
Dig two 1 foot square, 6 inch deep holes, 1 foot away from each other, in this case. If your stone slab is longer, space the holes accordingly. Place four stone pillars in each hole, seating each pillar in one corner of each hole. They should fit snugly together in the hole, with the sides pressed together.
Fill each hole with mortar to keep the pillars in place. Allow the mortar to cure for about 72 hours. This ensures the mortar is completely hard before you set any weight on it.
Squeeze a spiral of liquid masonry caulk over each set of stone pillars. Available at hardware stores, masonry caulk bonds stone pieces together very strongly.
Place your long stone slab on top of the four stone pillars in the holes. Adjust it so the edges of the long stone slab line up with the edges of the stone pillars. Allow the caulk to set for 48 hours before sitting on the stone bench.