1. Try to break up your huge application into modules that are functionally independent.
2. Within each module you start with the functions one by one.
3. For a simple function write all possible test cases that arise to be tested, while using the application as there are no specs.
4. In this way you could complete one function and in turn whole application.
5. To prepare test cases or plan make use of Excel sheet. Each sheet will define each function within the module. This is best way to organize the test cases.
