Hi m8,you have to put the belt round the motor pulley first,then put the belt as far round the drum pulley as you can,then cable tie the belt to the top of the drum pulley and spin the drum by hand,thats bout the only way to do it!!!.
Worth mentioning that if the belt has come off then either the belt is worn ( stretched ) the bearings are shot or the motor mounting brackets have snapped on the drum. If the belt has just come off and the drum bearings are nice and quiet then you really need a new belt, if it has come off once it will do it again.