Right from the beginning, after exchanging greetings, pleasantries and chit chat. When they start the interview by asking the first question, that's when I do it. I say something like, "Before we get started, I just wanted to let you know that I stutter so if you hear me take a long pause, I'm just trying to get the words out." And then I might mention that I'm actively working on my public speaking skills and trying to get better. But of course there's many ways to say it.