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Start by reading the requirements carefully and sketching ideas in a notebook or on a whiteboard together with a colleague. When you have understood more about what’s needed, note down some test cases you’ll need. Explore a few different avenues and aspects of the problem and expand your test case list. Analyse the list and pick the simplest, smallest one to begin with. Open your code editor and write that test.
This is an excellent way to begin working on a new piece of code. Your aim is to understand enough about the requirements that you find a good starting point. Your first test case should be simple enough that you will make it pass quickly, then you have some ideas about where you will go from there. You won’t do really comprensive analysis until you’ve written some code and know a little more about the problem.