One of the interesting parts about the breath is that it is involuntary. This means that if left alone, the breath comes in naturally. If there is any attempt to "control" the breathing process, this amounts to tension that interferes with the process.

Once natural breathing is allowed to happen and the voice hooks up with the intercostal expansion (that occurs naturally) then some measure of allowing the voice to phrase based on this support can be somewhat controlled.

The process of breath support and the relaxation of the facial muscles, vowel placement, relaxed tongue, flexible jaw and elimination of inhibitory tension in the mask has to be dealt with by a good vocal teacher.

Posture is important but not a rigid military stance. The shoulders and chest should be relaxed but up, not collapsed. I think in bel canto they call this the singer's "noble" stance. No inhibitory tension while standing this way.

The extremes in vocal flaws are too much breathiness or belting, both injurious to the cords.

Once the breath support (involuntary process) is available, the right amount of air goes through the cords which approximate and produce a solid natural sound. This then needs to be developed through exercises that free the sound and not inhibit it.

It's tricky. There's a lot of psychology that goes into it. Alexander Technique is good. The best is to find the best vocal teacher you can by listening to students who study with him/her.

I refer you to NATS (National Association of Teachers of Singing) which can be found on the website.

Get a good teacher.

Frank