now remove the protective plastic foil off the acrylic. you can add some felt to assure some spacing between the hot laptop and the plastic. this will allow some of the air blown by the fan to cool the rest of the laptop's bottom.
the book you see in the picture is the final touch. it's giving you the inclined surface you need to type comfortably and allows the fan to pull/blow air. i know you can add another piece of acrylic instead of the book but this is really meant to be cheap and fast :)