When I hear a voice that has any issue: that is tight, or flat in tone, the first place I focus on is the jaw.
In fact, that the jaw is such a terror, such a culprit of vocal tension, a detrimental demon to tone, flexibility and range that I've seen this wonderful exercise transform a singer in seconds.
And it's so easy.
First: Take two imaginary golf balls
Put them between your back teeth.
Why does this work?
Your larynx hangs from your jaw by muscles and ligaments. When your jaw is tight, it pull on the larynx - not allowing "your instrument" "your voice box" to move easily and to vibrate as a sound chamber wants to do.
Your jaw hinge is a ball and socket. By placing the imaginary gold balls between your back teeth, the ball and socket joint is instantly loosened, letting the larynx hang free.
Of course, loosening the jaw also allows more space in your mouth: your "acoustic room on your head, so those lovely sound vibrations can bounce and ricochet around the space.
I hope you enjoy this exercise and please let me know how you get on.