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To build the frame, place the 36-3/4 inch 2x4s between the top and bottom pieces. These will become the studs of the wall. Because one end will have the support of the wall and the other end will not, it is best to support the free standing end by positioning the studs closer together. Attach the beams using 3-inch finishing nails.
Once the frame is built, position it next to the wall where it will attach. Measure the bottom of the frame and make a mark at the 3-foot point to accurately position the frame so it sits square. On the perpendicular wall, measure and make a mark at the 4-foot point. If the frame is square to the wall, then the angle from the 3-foot mark to the 4-foot mark will be 5 feet exactly. Adjust the frame until you get this "3-4-5" measurement.
Once the frame is aligned, attach it to the wall and the floor using 3-1/2 inch screws.
Add a second 2x4 to the top of the frame. Once in place, set the level on top. If everything is level, secure the 2x4 with 2-inch finishing nails. If it is not level, place shims underneath until it is, and then nail down
Now it is time to add the walls. Using MDF is a quicker way to create a basic wall. (If the wall is going to contain electrical wiring within, then dry wall must be added to meet fire code standards. Check local codes in your area for details.) Add the 3/4-inch MDF to the sides and attach it with 2-inch finishing nails. Then add MDF to the edge of the wall and attach it with 3-1/2 inch nails.
Once the wall sides are up, add the pine top plate. This will become the knee wall top. Cut to 12-inches wide and however long you wish it to be. For this project, we extended one edge of the top creating a wall ledge. Attach with 2-inch nails.
Now attach the crown and base molding to the top and bottom. Measure the molding out and use a miter saw at the ends to cut a 45-degree angle. This will create nice clean corners for the molding. For the top, add the molding underneath the pine beam to bring the top and the wall together.
Use a sander to even out any tough spots in the seams that fall between the pieces, then use wood putty to cover them. Once the putty has dried, paint the wall to match the rest of the room.