Useing a lighter.
It is a quit normal behaviour when your bird starts biting the nappy suit, it’s getting less once s/he got used to it. Because of that, sooner or later you will have loose threads on the edge of nappy suit.
This is how I mend them. I don’t cut them with a pair of scissors, instead burn them with a lighter. I think burning will stop making the sharp edge which your bird easily finds for another biting.