Found the answer on diynot.com:
The procedure for fitting this type of belt is loop over motor spindle first then up to drum pulley wheel and fit as much as you can around the pulley. You will find it will probably go about half way round. At the point when the belt will not go onto the pulley rim any further,wrap a cable tie around the pulley rim and the belt and pull tight.Slowly turn the pulley wheel clockwise and the belt will engage with the rim after half a turn. You may still have a problem as the belt should not have come off in the first place which may indicate a fault with the machine.
JohnF
May 2007
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