There are several ways to train the voice

  1. Get a good voice coach. He/She will give you some exercises to be done every day.
  2. Sing in a choir with a good voice coach and practice at home with the same exercises. But note: This is good for the choir-sound, probably not  for an evolving soloists voice.
  3. For me as a professional it is possible to invent my own exercises on the base of experiences with my voice teachers.

In my opinion it is necessary to separate warm up, learning a new technique and studying a piece of music.

Each of this subjects are essential. Here I can only give some hints, because the relation to a good and matching voice trainer is very important to succeed.

Warming up

  1. Do some exercises to combine your breathing-in  with the erection-muscles of your body. This will have the effect of a natural support. Activating the breathing -in-muscles is necessary. After every breath sing immediately a short phrase in a third ore fifth range. and keep going. To get an effect every exercise has to be repeated at least 3 times.
  2. Combine  forming the vowels first with breathing in + phrase then with breathing out +phrase.
  3. Use round vowels first. They support the resonance.
  4. High tones have more open vowels than lower ones.
  5. Beginners have to start with  middle high notes singing the scales downwards to prevent you from regulation the pitch with using air pressure. If you want to reach higher notes, you have to get softer and softer, because for the chords it is very similar to sing high notes and piano notes.
  6. Keep going! Don't control too much! Just do the exercise! 
 

Learning the technique is completely different:

  • Guidance of a trainer is needed.
  • The trainer is not only for giving order but to guide your concentration and your attention to the important aspects. 
  • Be aware of what your body is actually doing. 
  • Hear the effects which  an exercise has to the sound of your voice.
  • Never start with complicated tasks. For example: Getting louder or softer is an intermediate skill. So it is with high and loud phrases with text.
  • Awareness -> Hearing -> Controlling ->Awareness...

Studying  music is completely different as well:

  • Never mix warming up and studying the music
  • When you want to start with a piece warming up is done. 
  • Be sure of the style in which the music is written.
  • Even most professional musicians need help learning the score with a répétiteur. Alert: If you had a good training, don't get confused with some of his advices. Try to find out what he/she wants achieve and  use the technique, you have learned.
  • A  répétiteur is not a coach. He only can introduce you to the music score. Everything else is your business including reading your part.