Buy a new one because if its been slipping its probably just run the teeth down, about £4. Then just undo the alternater holding bolt and the alternator will move forward, then slide the belt off, to put the new one on hook the top of the belt round the alternator and then bottom bit half way round the other wheel (forgotten the name of it) and then put it in first, rock the car back and forth and it will pull the belt on itself.
Then just tighten the alternator, you dont want it too tight, just want it so there isn't any give in the belt when you pull it, get it right on that limit and dont over tighten or you get a whinning from the alternator.

taken from another thread

alternator belt yes
look in haynes manual yes

:P

cool, how do i do that? Cheers

it will probably be your alternator belt... you might need to adjust the tention on it a bit

when its been freezing and i rev my car, i get a horrible squealing sound, that i also get wen its wet and i go round corners and increase my revs, is this my cambelt? If so, how do i fix it?? Cheers