how to train voice while just singing
Just listen to me singing, and do anything BUT that. LOL!
I have found that when I am alone in the car, when taking a shower, or singing a duet with one of my dogs, my singing ability increases exponentially with the volume of my vocalizing. Of course, no one else seems to share this opinion, but then I rarely listen to the critics.
ascending major intervals......Me aye me aye me aye me. Up one tone...repeat...up one tone, repeat. descending tone...as above Now let's do thirds... Ok, now let's work on fourth and fifth slurs... erm...best to join a choir. The structured practice you get is worth its weight in gold.
If you websearch for voice teachers, many of them have posted some basic advice on how to care for your voice, so you can at least avoid damaging it. Beyond that: Practice, practice, practice. When you reach the point where you can no longer teach yourself, or can afford to, have someone who Had A Clue listen to you and help you fix your weak points. Practice more. Voice is a musical instrument like any other. Some play it better than others.