Measure the length of the dividing wall in the room where the wall will be built. This measurement is from where an existing wall is to the edge of an opposite wall or into the center of the room.
Place a length of painter's tape along this measurement. This is where one edge of the new wall will be.
Measure the width of your base plate, plus any drywall, from the first tape line. For instance, if you are using a standard 2-by-4 piece of lumber and 1/2-inch drywall, you would add 3-1/2, 1/2 and 1/2 to give you a width of 4-1/2 inches. This is because 2-by-4 boards are not really 4-inch widths. If you use larger or smaller materials, then adjust the math accordingly.
Place another strip of tape parallel to the first one at the determined width of the wall. Use a utility or carpet knife to cut along the inside edges of the tape lines. Remove the cut section of carpet. Now you can install the base plate for your dividing wall and continue framing.