Its incredibly easy.
Carefully lift off the platter taking care not to damage the bearing if the bearing outer lifts with the platter. If you do remove the bearing outer it may be worth checking the bearing lube. Place the replacement belt around the inner sub platter. Thread a bit of cotton thread under the belt. Carefully lower the platter back onto the bearing. Pull the cotton slightly to hook the belt over the motor shaft. The platter should be eased carefully down to stop it "dropping" sharply.
Voila, the belt is fitted.
Sorry no pics its so easy though!