Most schools will explain what they want you to do on their school's profile description in AADSAS.
If no directions were given, it's easy to find some school's supplemental fee while navigating on their website, which you can then do (just make sure to read their supplemental website page to make sure they don't have specific instructions on when to pay).
Otherwise, most schools will email you letting you know that they received your application and that your supplemental fee is now due.