I usually start with a REPL, then play around with that for a while. For languages without a REPL I usually just create a one time file in the project and play

I usually start with a REPL, then play around with that for a while. For languages without a REPL I usually just create a one time file in the project and play around with that. Once I am a bit more comfortable with the code and have a better understanding of what I need, I will start writing some tests. I usually don't write too many early on, this way I don't have to go back halfway through development and change all my tests. Only when I'm confident enough with the code do I start writing extensive tests and try to cover all cases.