Measure the width of your carpet runner and subtract 1 1/2 inch. This is measurement A.
Count up how many pieces of tackless strip you need. There should be a tackless strip for each stair step.
Cut as many strips as you counted in step 2 using sheet metal snips, making each strip as long as measurement A.
Fit each strip to the place where the riser meets the tread below it, centering it as you do so, and nail it into place.
Measure the depth of each tread, which is the distance from the riser in the back to the edge of the tread in front, and add 3 inches to the measurement. This is measurement B.
Cut rectangles of carpet pad that are as wide as measurement A and as long as measurement B.
Butt one wide edge of the carpet pad flush to the first tackless strip and secure that edge in place using a nail gun.
Flatten the pad over the tread and bring it over the edge to the next riser, using a nail gun to secure it there as well.
Repeat steps 7 and 8 until all the stairs are covered with carpet padding. At this point, you are ready to lay your carpet runner.