Chipboard is a form of particleboard, which means it's pieces of wood and other materials that are pressed together with glue to form the boards. It's often used as an underlayment for wood, tile and sheet flooring. Tongue and groove chipboard is made to fit tightly together to form a solid floor base.
Lay a sheet of chipboard in one corner of the room, with the long side of the sheet running along the longest wall. Leave a 6 mm (1/4 inch) space at sides of the board by the walls.
Drive screws every 15 cm (6 inches) over the whole surface of the board. Make sure the screw heads are fully sunk.
Place the next sheet of chipboard at the end of the first, locking them together via their tongue and groove fittings. Get them tightly butted together, while keeping the 6 mm (1/4 inch) space at the wall. Secure with screws as before. Repeat, laying as many full sheets of chipboard as will fit along the wall.
Measure from the end of the last full piece of chipboard to the wall. Cut a piece to that size, using a T-square and pencil to mark a straight line and a circular saw to cut. Lay it with the cut side facing the wall, so the tongue and groove side can connect with the other boards. Screw it down.
Start the second course so the ends of the boards won't line up between courses. Cut the final board as needed.
Repeat, covering the entire floor. Install your top floor directly over the chipboard.
Use only high-density chipboard, which is more moisture resistant than other types. Do not glue down the chipboard.