Design an Accounting System classes and methods (Models and Services) in Dot Net [closed]

I am working on an Accounting system design problem and I would appreciate any help or ideas to design my classes, properties and methods. I am not looking for any code implementation of functionalities but just the design layouts.

I have to design keeping following things in mind:

• The goal is to design the classes and methods (Models and Services) required for an invoicing system for IT companies.
• Company has multiple customers.
• Customers have one or more projects that are being executed by the company.
• Each project can be run in one of two billing modes:
• Time & Material – Hours are logged by people working on the project and client is billed based on a fixed rate by the hour.
• Milestone Based – Project has milestones and clients are billed a fixed amount when a milestone is completed.
• System should have the ability to generate invoices for customers and email them to customers at the end of every month.
• We need to keep SOLID principles in mind and also focus on making code loosely coupled.

Thanks For investing your time in this.