2 ways  to fit a drive belt
1) stretch version 
2) adjustable swivel gate on motor  
1) the belt is placed centrally on the  small pulley then fed on to the large pulley wheel up to the point where belt looks to small to fit (usually the last 6 inches) you now grab the large pulley arms (with gloves) and now turn the large pulley wheel to make the belt STRETCH  on the last bit.
2) the belt is placed around the large pulley the motor  gate and motor bolts  are slackened then MOTOR pushed inward  the belt is then fitted around  the little pulley, the belt is then  re-tensioned by pulling the motor outward  you  tighten the gate bolt then tighten the motor bolts. 
this is same as adjusting a dynamo/alternator belt on a car
                
 wirecutterman
                
 October 2010