Measure for the total rise of the stairs. Lay a board flat on the deck sticking off the side to the point of the intended landing for the stairs. Lay a level on top of the board to ensure it is flat to the deck. Measure from the top of the board to the ground. This is the total rise.
Divide the total rise measurement by 7. The average rise for stringer stairs is 7 inches. This determines the number of steps. For example, 28 inches total rise divided by 7 equals 4 steps needed. If it does not divide equally, round the number up for 5 or more stairs and down for less than 5 total stairs.
Cut a 45-degree angle at one end of a 2-by-12-inch board. Use a carpenter's square to determine the angle and cut with a circular saw.
Lay out the cut lines for the stringer. Beginning at the longest point on the cut end, measure out 10 inches. This is the first step. Lay a carpenter's square at the end of the measurement and align so it reads 7 inches at the edge of the board. Adjust the other side to read 10 inches at the edge of the board. Trace along the outer edge of the square with a marking pencil. This completes the rise and second step. Move the square and repeat the action for each of the determined steps needed.
Cut a 45-degree angle at the end of the drawn stringers. Cut back toward the top of the board, on what will be the underside. This will be the landing for the stringer.
Cut out the stringers. Use a circular saw to cut along the drawn lines from the top side of the board. Use this cut piece as a pattern to cut a second stringer.
Press the top angle of the stringer to the supporting board of the deck and attach with L-brackets. Place an L-bracket on each side of the board and secure with the included hardware. Measure the desired width of the stairs along the supporting board and attach the second stringer with L-brackets. Use a level to ensure the tops of the stringers are even.
Cut 1-by-6-inch boards to fit the width of the steps and attach to the riser area. Use 3-inch nails at the top and the bottom of each side to hold it in place securely on the stringer.
Cut enough 2-by-6-inch boards in half to make 2-by-3-inch-wide boards. You will need one 2-by-3-inch board for each step, so if you have 4 steps, you will need to divide 2 boards. Use a measuring tape to mark the boards and a level to draw a straight line down each.
Note: You may have these cut at the lumber yard if desired.
Cut the 2-by-3-inch boards to the width of the steps. Cut one 2-by-6-inch board to the same width for each step.
Nail a 2-by-3-inch board on each step flush against the riser. Use 4-inch nails on each end. Place a 2-by-6-inch board flush against the 2-by-3-inch board to cover the step area and use the 4-inch nails to secure.
Apply a wood sealant to protect the new stairs. Use a paint brush to apply a sealer on all sides and areas of the steps and allow the steps to dry thoroughly before using.