[ArrayBoundCheck]

First you need to be experienced enough to know what not to do. Like that you shouldn't write mocks, shouldn't test private functions, shouldn't use a ton of global or static vars etc

Then you should find some sort of coverage tool so you can know if you covered most lines or not. I tend to ignore error handling lines but if its easy to get into that case I'll writ a test for it. Then I'll write test that take <1ms to execute. If something needs a network I'll try to separate the logic so it can work on a file OR I'll it connect to something on my local machine.

Generally I try to get 95%+ coverage (including error cases). I have many test and the entire suite run in <100ms. Maybe my project is easier to do TDD on but that's the way I do it