Releasing package/library into production?

Whenever you finish writing a new package/library/bundle in any language what you do next until you put it in action either for the open-source community or your company apps?!

I mean once you finish writing it what do you do next, how do you do the testing, do you put it in an actual project or a clean project to test it and see it in action how it works or you just rely on the unit tests you write for it or what exactly?