Position the wood posts on the side of the top and bottom stair treads, leaving the same amount of room from the front to the back of the stair treads. Trace around where the posts sit on the stair treads and then put the posts aside.
Drill holes in the stair treads at the corners of the tracings you made. Insert a jigsaw’s blade through the holes you drilled, and cut along the lines you traced. Insert the posts into the holes you cut, and hold torpedo levels against the top and side of the posts, positioning the posts until they sit level.
Drill holes through the side of the staircase and into the posts. Insert a bolt through the opening, and then slide a washer and nut onto the end of the bolt, located under the stair treads. Tighten the nut on the bolt, using a socket wrench.
Measure from the stair tread at the base of each of the rails, and mark a line at 36 inches. Hold the stair rail against the sides of the posts, lining up the bottom edge of the rail with the height marks you made. Trace where the bottom edge of the rail sits on the posts, and then remove the rail.
Cut the posts with a handsaw, following the lines you traced previously along the bottom edge of the rail. Lay the rail back on the posts, and drive four finishing nails through the rail and into each of the posts.