Appoggio and sostegno are two phases of singing on the breath. When you start to sing attack the sound on the breath to obtain appoggio, then you will obtain sostegno in the moment of maximum pressure (that has to be gradual, see above: exhalation).
If you don’t have appoggio, you won’t have sostegno. To easily sustain the sound until the end of the phrase imagine the phrase or vocalize as a whole, don’t divide it in syllables or notes.