Building a wall in place usually makes sense when the ceiling you're working with is covered with plasterboard. You won't damage the ceiling by forcing the pre-built wall against it. This method of adding an interior wall also makes it easier to adapt to existing walls, floors and ceilings that aren't quite square.
You'll begin by marking the position of the new wall as described in the last section. Cut your bottom and top plates and mark the position of the studs. Have a helper hold the top plate against the ceiling while you nail or screw into ceiling joists along the line. Attach the bottom plate to the floor directly below the top plate.
Measure the distance between the two plates. If the floor or ceiling is uneven, be sure to measure the exact distance at every stud. Cut each stud so that it fits snugly between the two plates. First, insert the studs at either end of the new wall. Correct positioning of the two plates should make them vertical, but use a level to make sure. You'll have to toe-nail (nail at a 45-degree angle) or screw the studs to the plate. Connect them with the studs in the existing wall if you can (your stud finder will tell you where they are). Continue attaching the rest of the studs in the same way. Metal stud-framing clips, which let you nail straight into both stud and plate, are an alternative to toe-nailing [source: Holmes].
Whichever method you've used to put the wall up, it's time for finishing. Measure 4 feet (1.2 meters) from the floor at each end. Snap a chalk line across all the studs. Install cross studs snugly between the end studs at this level. "They give your wall some extra support in case someone bangs into it," says contractor Donald Holmes. "And they provide a place to attach your plasterboard."
You'll finish the project by attaching sheets of plasterboard to each side of the new wall with nails or screws. Then you'll apply plaster-like joint compound and paper tape to each joint and corner. Sanding and painting are the final steps. Voila: where you had one room, now you have two.