How do you write Apex test classes correctly?

Requirements:

1. Class annotated with @isTest.
2. Methods annotated with @isTest (or testMethod keyword).
3. Minimum 75% code coverage across all Apex before production deployment. Test data must be created in the test (not relying on org data) unless @isTest(seeAllData=true) is used (avoid this). Key patterns: @testSetup method runs once and creates data shared across all test methods in the class. Test.startTest() and Test.stopTest() create a new set of governor limits and force async operations to complete. Assertions: System.assertEquals(expected, actual, message), System.assertNotEquals(), System.assert(condition, message). Test for bulk (200 records), single record, and negative cases.