Start with a clean belt. Use water and soap, rinse and let dry.
Put some talcum powder on the DRY belt.
Get the rearwheel off the floor, if you have not done so previously when cleaning the belt.
Thighten the axle so much that the adjusters can be adjusted with a spanner ( not by hand).
That way you eliminate the cushioning of the rubber dampers in the rear pulley.
Tighten 1/4 of a turn evenly left and right while turning the rearwheel.
Observe the belt running in/on the middle of the pulley.
Stop tightening ( the adjusters) when you can twist the belt to 90 degrees with your fingers.
Torquetighten the rearaxle and the locknuts of the adjusters.
Lower the bike and your done.
Works fine for me. If the belt squeaks, mostly in the rain I don't bother. If you can't stand the noise, wax the SIDES of the belt.
Greetz
Kropatchek