Hi Steve If this is something you have not done, then I'd agree with Mike and suggest you hire someone who does this kind of thing all the time.
FYI though, step carpet, whether a runner or full carpet, can be of two varieties: contour or waterfall. The former is usually done with carpet tacks and follows the contour of the stairs from top to bottom. I've seen this only on very thin, single layer carpet or mat. The latter is what is normally done. It goes over the protruding bull-nose edge of each tread, straight down to the corner of the next tread/riser below. It is held in place with tack-strips (1" or so wide 1/4" lath type boards with sharp little tacks sticking out that catch and hold the carpet in place). This is the better and safer way to go. The potential safety problem here is that if the carpet is not securely held down, over time little carpet pookahs will stick up, just asking to catch a toe on someone hurrying down the stairs, who will then do an involuntary superman-style dive, but unlike superman, will not get up without a scratch! Caution is the word here :-)