Business Requirement / Specification document helps in developing test cases.
Using BR (business requirement) create Use Case diagrams to create scenario's then split that into multiple test steps to execute the scenario's
Open source tool is available for use case diagram too....
Try that to make it more effective test scenario's to meet the BR (Otherwise called Spec) using use cases
As tester job is to take positive & negative scenario's and security related based on the application which your going to test.