Stair and rail related carpentry is about as advanced as it comes. If a post isn't fastened correctly then you will regret it every time you pass by, grab it, and it wobbles. There are many ways of attaching them, mostly dependent apon exactly what the framing conditions are below it and/or next to it. Posts are not attached to the finish floor, but to the framing in the floor and/or stairs. My latest and greatest cheat/trick was to drill a hole with a spade bit and extension that goes deep into the framing below, drill the same hole in the bottom of the newell post, and use a lg. threaded rod (3/4"+). The hole has to be the size of the shank and the trreads cut into the wood around the hole. This was done in an existing, finished situation where I didn't have good access to framing. I waxed everything up to make it go easier. It worked great!