What I’d say is the most crucial when it comes to the singing voice is to make sure that you don’t over-adduct (or push); meaning that you don’t squeeze together the vocal folds excessively. This usually happens when you sing high notes, when you want to reach high and subconsciously, you end up pushing too much in your throat.
So here’s an exercise to avoid this common error. Try humming In the NG position as in the end of the word ‘hung’, which will make you place the back of your tongue to the soft palate. Keep the back of your tongue connected to the soft palate and while humming, imagine that you’re chewing gum. Do not try to make a big sound. Just let the sound happen by blowing air. If you do this exercise, usually you will not push or squeeze excessively. You will have that ring in your voice without any strain or pushing.
Do not try to hit high keys with this exercise, as you cannot reach very high in this position. I’d say not higher than the low B or middle C for higher voices. See for yourself which note you still feel free with your voice. After doing this quite a few times, open your mouth and try to use your voice in the same position until you get used to producing sound without any force – just as if you’re blowing out air.