One exercise to start with is to try to practice speaking in front of the mirror. You can use any texts or even just say the alphabet. Notice how the muscles in your face move. Then do it again several times and think about opening your mouth more and exaggerating with each sound to feel and see where in your mouth and which facial muscles you need to utilize more. The more you work out these small tiny muscles the easier it will be to have clearer diction and not mumble.
