How do I tighten the belt for my Hotpoint 9330 reversomatic tumble dryer?

We've managed to fit the belt over the drum and the pulleys but it is still slightly slack how do you tighten the belt so it is snug to the drum. Can you also let us know if we have fitted it correctly, (it appears that we've done it right but you never know as the drum and belt is going round with the pulleys) so could you just give us instructions how to fit the belt as well for future reference just in case it goes again.