not all make's of machines have this facility, so you will need to stretch the belt on.
place belt centrally on motor pulley, offer the belt on to the large pulley, there will come a point where it looks that the belt is too small to fit ,now start to turn the large pulley( via the center arms) the belt will stretch and jump on , give it a couple of turns to see that the belt stays on.
this is like putting a chain back on a bike
wirecutterman, March 2010