Keep trying until you get it. I started this recently, instead of substituting words. At first, it was awkward and prolonged. D...d....dddddooo...dd...dog. But, with time, it has gotten easier. The gaps, stutters and blocks are smaller. That's not to say my stutter is gone completely, not at all. But when asked a direct question, over about 3-4 weeks, answering instead of substituting has seemed to work for me, at least.