I loved Mrs. Miller. Part of the problem with her sound was that she was nasal. Now, if you want to stop that sound and get as much on key as possible (sometimes "bad" singing may just be emotional singing). When you sing open your mouth as far as possible (if you hear a clicking sound, go to a doctor for TMJ). Take a deep breathe with your diaphram and sing. To keep from going flat, smile, smile, smile because it opens your resonators. Imagine the sound coming from your forehead. Open your eyes wide and look up. That should help. If you still need help (and don't we all), I suggest a voice teacher (and, as I have mentioned before, not a vocal coach).