Decide on the single thing you care about the most and make it work
Write a test for that important case
Sketch out many other tests for cases you think could be important
Rewrite the code two or three times while filling in the tests
Crucially, if you don't do #3, you can never do #4 because you won't know if your rewrites will be correct. Tests are insurance against making mistakes. They take time, but they let you move faster ultimately because you can be more confident in taking risks, which is the whole point anyways.