Once you’ve removed the old panels, start on one wall and work towards the opposite wall. Leave an expansion gap between the wall and floor using shims. You can conceal it with skirting boards when you finish.  The boards should lay the opposite way to the joists. Flooring joists typically span the shortest distance for rigidity and structural integrity.  Work in strips, completing one entire row at a time before moving on to the next line of chipboard.