While you're working, be sure to take ordinary safety precautions. Wear goggles and ear protection when needed. Get help lifting.
Start by marking precisely where you want your wall. Measure to make sure each end of the wall is the same distance from a parallel wall. At each of these points, place a carpenter's square against the existing wall at ceiling height and draw a line out a few inches at a right angle to the existing wall along the line where you want the edge of the new wall to be. Tap a nail into each of these lines. Stretch a chalked string between the nails and snap it to make a line. This will be the top edge of the new wall.
Now drop a plumb line -- a string with a weight on the end -- from each nail. Mark the spots directly below the top line. Snap another chalk line between these points. This marks the bottom of the wall. Attaching the wall to the floor will be easy. You can attach it directly to almost any kind of flooring except carpet.
If the plasterboard has been removed, use a level to find out if the existing walls, floor and ceiling are square. If so, you can make life easier by building your new wall on the floor. First, cut two plates, the boards that will form the top and bottom of the wall, from 2-by-4s. They will run the entire length of the new wall. Lay them side by side. Mark the locations of the studs on both plates. There will be one at each end. The rest should be separated by 16 inches (41 centimeters) [source: Gibson].
Measure the height of the room, and then subtract the combined thickness of the two plates. This will determine the length of your studs. Cut the studs, place them between the top and bottom plates, and attach them with 3.5-inch (9-centimeter) nails or screws.
With a helper, lift the wall into place. Line it up with the lines you made on floor and ceiling. "Sometimes people set the bottom along the line, then push the top into place," says Donald Holmes, a Milan, New York-based contractor. "But it's often easier to line up the top of the wall, then tap the bottom into place using a sledge hammer and a piece of scrap wood to protect the plate."
Apply a level to make sure the wall is exactly vertical. Shim any gaps between the new wall and the ceiling. Nail or screw the wall to the floor and to the ceiling joists. If the new wall aligns with a stud on the adjoining wall, attach it there as well.
Your wall is up and ready for finishing. But what if you're working in a space that's finished or where the walls are not exactly square? Then you'll want to build the wall in place. We'll talk about that method in the next section.