I want to finish an attic in an older home so I would like to install knee walls to close things in. Do I install the knee wall first and then lay decking up to the wall or do I lay decking first and install the knee wall on top of that? Thanks.

You need veticle studs behind the knee wall so do that first, then it doesn't matter which comes next. I would do the flooring but there is no rule.

Ok, showing my ignorance here... I need verticle studs *behind* the knee wall? I was going to rip a top plate to the proper angle so my drywall would be flush and then just build the wall, stand it up and nail it in. Is there something else I need to do? I've got about a dozen home repair and framing books and I couldn't find a single thing about how to do this.

No John you can do what you describ. That is how we build a wall normally but with a knee wall we frequently tie into what's already there, the rafters and the floor joist. Either way is good.

Put in the sub floor first, if the area is used for storage add additional flooring as needed
With the sub floor below the wall framing you have a full bottom plate exposed for drywall fastening and wood baseboard attachment, and frame wall as you described :)

Thanks guys. I'm looking forward to getting started on this.

Let us know how it goes, we are interested in outcomes.