Light touches. When you say words that start with "m", barely press your lips together. Do not emphasize the "m" sound, just barely press your lips together, just hard enough that your lips touch. On "D", you might be causing tension in your throat and in your mouth. Purposefully make yourself stutter on the letter D, and pay attention to what that feels like. Relax the muscles in your throat and mouth. Barely press your tongue to the roof of your mouth. Again, just so the tip of your tongue barely touches the roof of your mouth. It can be hard to implement things like this into regular conversation, so start slow. Start by yourself reading aloud. Move on to talking to people with the specific intention of using these technique.