I think "hard" is an understatement lol. You won't be able to do it in difficult situations. You have to start with the safest, easiest situation in which you block and tell yourself, "my goal here is to make a sound on the sound I don't want to say". The trick is finding a situation where you feel willing to let yourself take the chance to prioritize that goal over all else (the conversation, the listener, etc). That might mean with the person you feel safest doing this with (where you still might block), this might mean going into a library and asking a nice librarian a question. Whatever you think is best for you to allow yourself the best odds of giving yourself the time to work on the goal of making the sound. You won't like how it feels but that's precisely the point. To do it enough times that you dislike it a little less and eventually you'll be able to stutter on the sound instead of blocking which is what you want. Again, all this is much easier said than done.