Measure the dimensions for the floor and mark them down on a piece of paper.
Cut four pieces of 2-by-4 lumber to serve as the outside frame for the floor. Make sure that each side is the same size as its opposite side.
Cut several more pieces of 2-by-4 lumber for stabilizing joists. These joists will go inside the frame, spaced vertically or horizontally at 16-inch intervals from each other.
Nail the joists to the frame.
Place the frame on the ground. Drive stakes in at each corner. Put the frame aside for now.
Place sand at each corner and place a flat paving stone over that. Use your level between paving stones to ensure that they are level with each other.
Lay a sand-and-paving-stone foundation between the two sets of corner stones, with one stone every 3 feet. Make sure all the stones are level with each other.
Cut two pieces of 2-by-6 lumber as long as the length of the floor frame. Attach these two "runner" boards to the bottom of the frame so that they rest on top of the paving stones.
Lay the floor frame over the paving stone foundation and make sure it settles properly into place. Adjust the sand under the paving stones as needed to make the floor level.
Cut and attach the floorboard lumber to the top of the floor frame. The lumber you use depends on your own tastes; you may use standard plywood sheeting, or you can also use 2-by-4s or similar kinds of lumber for a more sturdy floor.