check the drum bearings are ok and NOT SHOT
check if the motor bracket has elongated slot to tension the belt if it HAS fit belt to the pulley then motor then apply tension to the motor to TENSION drive belt (like adjusting alternator belt on a car)
if no slot apply the belt squarely on motor pulley now feed the belt onto the pulley ,there will come a point where the belt don't look like it's going to fit at this point turn the large pulley until the belt jumps on .
wirecutterman, August 2009