fitting simpson sirocco 455 drum belt?

I have overhauled my Simpson Sirocco 455 tumble dryer and can not see how to track the drum belt which is new and of the same size as the original. Both the motor and idler pulley are fixed so where is the tension created? Fitting the belt, the way I think it should go, ends up with a loose belt.
It looked so simply when I pulled it down that I did not take photos!!!!
Can anyone help me please as it is driving me nuts???!!