Breathe deeply, filling your whole abdomen with air. While you may be used to filling your belly with air, you should take in enough air to expand around your back as well. Practice this by sitting or standing straight. Breathe in, while counting to eight, then breathe out fully for eight counts. You will feel air fill the space around your spine, if you draw your belly button in on the exhale.
Lift your voice from your diaphragm. Contract and expand your stomach. As you sing, push out with the air in your stomach and project your voice out. It may be helpful to keep your hand on your stomach to feel these muscles while you sing.
Stretch your mouth vertically wider when you sing. Open your mouth taller and drop your jaw. This simple act can help divert your tendency to sing through your nose. It will also encourage you to take deeper, fuller breaths by filling up your lungs with air. This act can also help strengthen your posture while singing.
Take a deep breath from your belly and plug your nose. Focus on breathing from the abdomen and out through the mouth. With your nose still plugged, try singing using this air in your abdomen. Your instinct might still be to breathe through your nose, so relax your jaw muscles and open your mouth wide to channel the breath away from your nose and out through your mouth.
Inhale through your nose. Once you have developed the ability to fully project your voice using air in your lungs and pushing out from your abdomen, practice breathing in through your nose while singing. If you continuously breathe in and out through your mouth, your vocal cords will become dry and this might compromise the notes.