First of all make sure the grooved side of the belt is lined up so that the grooves match those on the spindle in the centre of the motor. You then have to pull the round black tensioning wheel up and over in a 180 degree arc. In other words, facing the back of the machine/motor unit, with the belt slack, the tensioning wheel will be at your bottom left, pull it over in an arc to the right-hand side [the motor is quite loose in the cradle]. You will then feel the tension on the centrifugal spring tighten up, and it will become almost apparent where the belt should loop over the spindle and the tension wheel. Once this is set up, the tensioning wheel will effectively tighten the belt. It takes a bit of fiddling and holding the tensioning wheel at the right whilst looping the belt through takes strong fingers, but hope this helps.
Bill
May 2005