In the case of Test Driven Development:

• Implement the unit test before the coding of the functionality is started
• All the code paths should be tested
• Add any extra test coverage on the basis of the technical design (where selected quality applies)
• Make the unit test independent of other tests, so that failure of one test case will have no consequences for other test cases
• In the event of a defect, prior to solving it, write a unit test that shows the presence of the defect (and the absence of it after solving it).