Late payment fees

Hi guys, this is my first post here and I was wondering if you could help. I'm interning and have been asked to create a system that would help them introduce late payment fees on their past due debtors.

They are able to charge 4% + NATWEST base lending rate per month overdue, compounded quarterly.

I am not sure exactly where to begin, and any insight into making something that could calculate the amount that could be charged would help.

Thanks!