Seems like I read something on this recently. Maybe in a recent issue of family handyman(woman) magazine. Anyone else remember that?
You basically start at the bottom and nail down your tack strips. Staple down the padding between the tack strips. Then use a hammer and a 6" wide heavy dull chisel to drive the carpet into the crack between the tack strip at the back of the first tread and the tack strip just above toward the bottom of the riser which causes the carpet to engage the tack strips. Then just work your way up. Seems like the magazine article didn't use a tack strip on the risers but instead just stapled the carpet to the bottom of each riser after making the corner with the hammer and chisel. I think the tack strip on the treads was nailed down about a half inch or a finger thickness (just like a wall) from the back of tread where it meets the riser which allows enough room to drive the carpet into the crack. Seems like they said not to stretch the carpet too tight as you bend it over the outside corner of the tread but then you don't want it too loose either.