Well, you must actually practice with good vocal exercises to perfect your singing breath. Singing from the diaphragm is a complex muscle/breath co-ordination that many singing teachers find it difficult to their students in words effectively. That is why there is so much confusion on this topic.
Just to give you an idea of how your diaphragm works when you are breathing, try this breathing exercise. Stand up straight with a relaxed posture, and then take a big yawn - Ahhhhhh! While you are taking that big yawn, notice how your tummy expands outwards? That is the diaphragm at work, pushing out your tummy so that your lungs can drop lower so that you can take in more air.
Now the diaphragm muscle is also used for the control of airflow by sending the correct amount of air to the vocal cords. As you sing the higher notes, less air is required for the vocal cords. By sending the correct amount of air, your voice quality will be stable and relaxed and the overall singing voice will sound so much better.
However, you need to practice with the correct vocal exercises until "singing from the diaphragm" is an ingrained habit so that you don't even have to think of it when you are singing because you are automatically using this singing technique. You don't even need to remember how much air to release to your diaphragm because you will be releasing just the right amount of air naturally.
Contrary to popular belief, you do not need breathing exercises for singing to strengthen your diaphragm! The muscles in your diaphragm are already strong enough already as it is. This is because your vocal cords are very small and delicate muscles and thus do not require too much support.
Since this is an article, it is not possible to show you the vocal exercises breathing technique demonstrations. However you can get a feel of it by experimenting with this exercise.
Now put your palms lightly on your tummy. Take in a deep long breath. As you are breathing in, extend your tummy outward and sideways as the air comes in. When you breathe out, pursed your lips together to release a controlled stream of air and at the same time, with your tummy becoming smaller and back to its natural state as air is being released. Now this is how you will feel when you are singing from the diaphragm.
