"The greatest musical instrument given to a human being is the voice".
Why warm up?
Would you run a 100m sprint without first limbering up and stretching the muscles? Of course not. Yet we often launch into singing or a day of speaking without warming up our voice. Warming up for 15-20 minutes is important before any sustained singing or speaking activity. An effective warm-up prepares our voice for the challenges ahead, whether it's an important speech, the start of your teaching day, a choir rehearsal, or a solo performance. The goal of the warm-up is to prepare for the range (highs to lows), the dynamics (soft to loud) and get the vocal folds working optimally. Personally, I sing a lot better when I do a good warm-up beforehand.
The first thing - stretch, energize and release tension!
The second thing - getting the natural breath going / unvoiced sounds
The third thing - voiced sounds that are natural, instinctive and organic
The fourth thing - open voiced sounds
The fifth thing - consonant energy