The pully comes in two halves and the belt tension is adjusted by removing/replacing the shims. To tighten the belt you will have to remove shims (put the ones you take off on the outside as you may need them next time). There is no science to this job, it's just suck it and see. Do it slowly one shim at a time until you have got it right. The shims come in a couple of differnt sizes so take out a small one first and try it. If that's not enough put the small one back and take out a thicker one. And so on.
Allan