I agree w/Henry. This isn't normally done. The sides of the stairs, or stringer, is usually left as is. I guess you can do what you want but I'd call a Pro. If I HAD to do it for some reason (which escapes me at the moment), I'd probably cut the pieces to size and just glue them on with Construction Glue. I know it won't look very 'nice' because you're gonna see the 'ends' of the carpet on 2 of the 3 sides. You might have to install some sort of 'trim' to cover it up. As Henry suggests, I'd just skip it.