[ppg960]

Your pulleys are running off center from each other. The belt needs to run evenly through the pulleys in the rotation. Alternator is misaligned,
tensioner is weak etc..
Take a straight edge and line up the pulleys across center from each other. They should all be equal. You will need a 13mm wrench to push the tensioner towards the front of the car and release the belt. It should be very stiff. If not, replace it.
On a 2.0 z-tec you need to remove the upper motor mount as well as support the engine at the bottom so it doesn't lean over. Remove the front wheel and the bottom of the fender liner ( 2 10mm bolts) to gain side access.
Any problems, re post.

Cheers.