So, how is this voice-building business done?
Experience has taught me that a multi-pronged attack is the best approach. I do not, therefore, rely only on scales, or even sound-making.
The voice is unique among musical instruments in that its structure must be established - and continually reestablished (i.e kept fit) - in addition to its being played. It is also uniquely susceptible, on a structural level, to intrusion by negative emotion.
That accounted for, its free-function also depends, to some extent, upon the fitness and flexibility of its foundation - that is to say, the whole body. Did you know, for example that simply locking your knees back (a very common habit) drastically interferes with vocal quality and ease? I check my students' knees as often as a driver checks their mirrors.
That said, it's mostly about the registers: chest (or full) voice and falsetto. You isolate them, you strengthen them, they start co-ordinating once they're strong enough and it's presto singo over about three octaves.
Finally we come to artistic expression and performance and the successful integration of the above into the art of singing and/or speaking well.
It is likely that, in addition to the exercises we go through, I will recommend you begin a good stretching regimen, a bit of cardio and the odd massage. I am also a great admirer of the Alexander technique. None of these are necessarily mandatory, but they are all helpful, and the needs of each student vary according to their particular condition.