Determine Step Size
Measure the height of the deck in inches. If your yard is not level, be sure to account for the change in grade.
One way to do this is to hold a long piece of straight lumber from the top of the deck (using a level) to over the landing area and measure from the ground up to the bottom of the board.
Divide the height by 75 (the average rise per tread) and round to the nearest whole number. The result is the number of steps you will have.
Divide the height by the number of steps to get the precise riser height ("x"). Be sure to convert from decimal to fractions.
Rounding to the nearest 1/8th inch is typical.
Draw Your Stringers
Locate the riser height ("x") using one leg of the framing square and the run (10 inches) on the other leg.
Place the framing square onto the stringer (typically 2-inch-by-12-inch lumber) so that these points both align with the edge of the stringer material.
Trace the resulting "V" onto the lumber. This will give you one step with a rise of "x" inches and a run of 10 inches.
Slide the framing square down along the lumber edge so that the "x" point aligns with the 10-inch mark of the previous tracing, then trace again. Continue doing this for as many steps as you need.
Complete the top step by flipping the framing square at the top tread tracing so that one leg is aligned with the 10-inch tread and the other leg runs parallel to the rise (this line runs from the top tread to the back of the stringer).
Complete the bottom step by extending the bottom line of the last rise backward to the back edge of the stringer (parallel to the tread).
Cut and Install Stringers
Using your circular saw, cut out the tracing. Be sure to stop at the inside corners.
Because the blade is round, the cut will not be complete, so complete the cuts with a jigsaw.
Install your stringers so that they are spaced 12 inches "on-center" (12 inches apart, measured from center of the board to center of the board). This will give your steps minimal bounce (deflection).
Attach the stringers to the deck with an adjustable slopeable stair hanger, a premanufactured connector found at your local hardware store.
Dig two holes 30 inches deep at the midway point of the stringer. Use a plumb bob to transfer this point from the outer stringers to the ground.
Fill the holes with concrete 8 inches deep and allow to dry.
Insert a 4-inch-by-4-inch post into each hole. The post should extend up beyond the stringer high enough to use as a railing post (typically 30 inches; check your local building codes).
Attach your support beam to the two 4-by-4s using lag screws or carriage bolts. Be sure the beam is snug against the bottom of the stringers.
Attach the decking and railing. Most building codes also require a riser kick plate.
This is a board attached across the riser (perpendicular to the tread) and is typically a 1-inch-by-4-inch or 1-inch-by-8-inch piece of lumber.