Watch this video from This Old House to learn how to make deck stairs.
1 Determine the width of each stair tread.
2 Use 4-foot level and tape measure to find the total rise of the staircase.
3 Divide total rise by number of steps to find height of each riser.
4 Set framing square and straightedge to height of riser and width of stair tread.
5 Use framing square to mark saw-tooth pattern of steps onto a pressure-treated 2x12.
6 Notch the stringer with a circular saw, being careful not to saw beyond the cut lines.
7 Finish cutting the stringer with a handsaw.
8 Use first stringer as a template to mark the remaining stringers.
9 Trim upper, rear edge of each stringer the thickness of the risers.
10 Trim bottom end of each stringer the thickness of the treads.
11 Fasten the first and last stringers to the deck frame with L-brackets.
12 Fasten the intermediate stringers with 3-inch decking screws driven through the deck frame and into the rear of the stringers.
13 Attach risers to stringers with 3-inch decking screws.
14 Install 4x4 railing posts with 3-inch screws.
15 Use jigsaw to notch treads to fit around railing posts.
16 Fasten treads to stringers with decking screws.