Another mental tool I use, borrowed from Uncle Bob, is the 4-A’s of testing.
Arrange (your initial state)
Act (perform the action that needs to be tested)
Assert (that the actuals meet the expected)
Annihilate (teardown your state if required)
In theory each test can be boiled down to 4 lines of code (in practice it doesn’t always work that way).
is pretty good talk on developing and refactoring tests. It’s Java but the principles are valuable.