I want to remove an existing railing around an opening 48 inches by 110 inches (leading to the basement) and replace it with a wall. If the wall will only be attached at one end and the other end (the 110" meaurement) is free standing, what do I need to do to make sure the end of that wall is not wobbly?
Since this is going to be on or next to the staircase, I would bolt it through the tread, stringer or riser depending on your configuration. Add a 2x piece of blocking to add to the thickness of the stair part you use.
To increase the lateral rigidity I would glue and screw a plywood skin on both sides of the wall framing. You could use 1/4" ply and cover it with 1/4" sheetrock to minimize the thickness.