Measure the area you want to wall off. Use a saw to cut four pieces of pine to the proper length and width or purchase pre-cut pieces that are already the correct size.
Screw together the four pieces of pine to make a frame for the wall.
Make a ledge by attaching moulding to the inside border of the frame using hammer and nails.
Put wallboard inside the ledge and attach it by hammering a second layer of moulding around the wallboard.
Bolt furniture levelers to the bottom of the wall. Ask a helper to hold the wall upright in the position you want it in. Attach it to the ceiling of the room. If the ceiling is angled, use double-stick tape like the kind used to affix posters to the wall to attach the wall to the ceiling.
Paint both sides of the wall so that it matches the bedrooms.