Wooden handrails for exterior stairs provide balance and support when going up and down the stairs, and set off the front stairway. Installing redwood posts and a wooden railing for concrete exterior wood deck flooring stairs requires special tools and experience with concrete anchors. For wood stairs, a do-it-yourselfer with basic carpentry experience can install redwood posts and a handrail at each side of a stairway using some fasteners and everyday tools. In a short time, the handrails and stairs will take on a new look.Measure the distance in inches from the lower end of the stairs to waterproof decking vinylthe upper end, and divide the measurement by 36, which is the spacing of the handrail posts. Measure from the lower end of a stringer at one side of the stairs, and mark the face of the stringer at 36-inch intervals. Count the number of marks, and add a post for the top of the stairs and another for the lower end.Hold a 48-inch long level vertically against the side of one stringer at a point that aligns with the outer edge of a stair tread, and make a vertical reference exterior plastic laminate line on the stringer. Determine the length of the posts by measuring from the lower edge of the stringer at the reference line to the surface of the tread and adding 36 inches, which is standard railing height above the treads. Note the length of the posts.