The objective of functionality testing is to ascertain if the behavior of the entity under test (EUT) is as intended. Behaviour of a EUT is about ’obeying some rules’. These ’rules’ are really the specification. So to test a functionality of any EUT, we need to first understand the intended rules and then create scenarios that are really various behaviors by combining the rules in different ways and then check the outcome(s).
To understand the intended behavior, first describe what the EUT is supposed to do. Describe the behavior as a series of steps. Each step accepts/uses data and processes the data according to some condition(s). Examine each step and extract the condition(s) hidden in the step.
Once the conditions are extracted from each step, re-write each step as a series of rules and what we have now is prescriptive behavior. From this it is easier to create the test scenarios and then identify the various data sets for each scenario and subsequently generate test cases and therefore for the entire EUT.