Calculate the rise and run that is needed for the deck stairs. This will tell you how many steps are needed in the stringers. Measure from the top of the deck to the ground and divide this measurement by the desired height for each step. Make sure the answer results in a whole number. For example if the top of the deck is 15 inches from the ground and each step is to rise 5 inches, then 15 divided by 5 equals 3. Three steps are needed for this example.
Place the 2-inch-by-10-inch board flat across two saw horses.
Position the framing square so that the thin side of the framing square meets the edge of the wood at the measurement that was set for the rise of each step. For example, if each step is to rise 7 inches, then the framing square should be placed so that the edge of the wood is line up with the 7 on the framing square. Use a pencil to trace along the edge of the framing square.
Line up the fat side of the framing square so that it matches the depth of each step. If the tread is to be 9 inches, then the fat side of the framing square should have the 9-inch mark at the edge of the wood board. Use a pencil to trace along the edge of the framing square.
Use a circular saw to cut out the stringer by cutting along the marked lines.
Cut the top and bottom of the stringer so that the riser butts up against the edge of the deck and sits level on the ground.
Attach the stringer to the deck with deck hangers around the bottom of the stringer. Secure the deck hangers to the side of the deck and the stringer with a drill and wood screws.
Attach the treads to the stringers with wood screws to complete the deck steps.