Why are my P5 serpentine belts squealing?

I have a 2002 Mazda Protege5 that had squealing serpentine belts. One was completely destroyed & other was fine but I changed both myself. Everything was fine for 2 days and then they started to squeal again (on and off) it mostly happens when I start my car and then it goes away, and randomly when I’m driving. Why would brand new serpentine belts be squealing on and off, and how can I fix it?
Thannk you