Measure the width of the stair tread and record it. Also measure the width of the stair runner. Subtract the runner width from the tread width and divide the result by 2. Using a pencil, mark this result inward from the end of each tread where it meets the riser.
Cut a tackless strip for each tread two inches shorter than the runner’s width using sheet-metal snips.
Spread a strip on a stair tread flush against the marks you made inward from the edge. Space this strip 1/4-inch from where the tread joins the riser. Hammer a carpet nail at each corner to secure it in place. Repeat this process for each stair tread.
Cut pieces of carpet pads three inches longer than the depth of each stair tread, but identical in width to the tackless strips. Spread a pad over the front edge of an installed tackless strip.
Insert 9/16-inch staples every four inches into the pad to secure it to the edge of the tread. Pull it taut while extending it over the front edge of the tread and insert staples underneath to secure it. Continue this process to secure pads to the treads.
Set a framing square over the upper edge of the runner and check if it is square. If the edge is not even, mark a straight line over the back of the runner and cut over it with a carpet cutter. Dab a bead of carpet glue over the cut edge to keep the fibers from coming loose.
Unroll the runner over the stairway. Center it over the topmost riser, with its edge taut against the nosing or stair edge. Center the edges of the runner between the pencil marks you make on the treads. Insert staples through the ends and every 4 inches in between the secure the upper edge of the runner to the riser.
Run your hand over the runner to the base of the riser. Press the runner to the tackless strip. Set a knee kicker over the middle of the runner and press it with your knee to tighten the runner to the strip. Move the kicker every three inches on each side of the riser and repeat the process to fasten the runner to the strip.
Insert a staple into the runner at each end where the tread and riser meet. Continue this process until you install the runner along the stairway.
Cut the runner at the last riser so it falls under the nosing. Apply wood glue over the edges of the runner and secure it to the lower side of the nosing. Insert staples into the edges to hold the lower edge of the runner to the riser.