driving me mad ! corsa 1.2 trip, alyernater belt keeps loosing tension?

Vauxhall corsa 1.2 p reg petrol. my sisters car, was squealing , then it stopped .When we stopped the alternator belt was gone, replaced it. about 7 days later it came back again, re - did the procedure 3 times, then went back got a belt that was the right one , it was only a mm smaller, but been ok , until now. Same problem as re-accured.
So now i think it me doing some thing wrong. can you tell me the politically correct way to replace the alternator belt?
Thank you and kind regards julie