You can buy an expensive tool (£25) but a neat trick is to use a metal tube, like a vacuum/hoover tube to place over the motor spindle and lever/stretch the belt once it's around the drum. Rotate the drum and tube to help slide the belt into position. A spare pair of hands will help but wear gloves as there are a number of sharp edges around!....PS unplug first!
Robski, December 2009