The Full Usage Case :

Welcome to my 3rd tutorial on Object Oriented Design. In this part of the tutorial, I’ll cover Iterative Development, The Unified Process and Usage Cases.

Unlike the waterfall process for object design, with the iterative process programming starts before all of the requirements are expected. The Unified Process is a popular IDP which focuses on high risk / value parts first before creating full requirements for the program.

The Usage Case in the link above will help you