As written in the sing for Dummies book
Knowing how to move the soft palate and coordinate that movement with your tongue is important for speaking and singing, because you want the soft palate to lift for a resonant tone. If the soft palate doesn’t lift, you make a sound that has too much resonance in your nose, or a “nasal sound,” as you may have heard someone say.
To make a sound that has a resonant tone, explore the following exercises to help you feel the movement of the tongue and soft palate in words. You can then apply that same knowledge to your singing.
When it’s time to sing a consonant that requires moving the soft palate, move the tongue and soft palate until they touch and then lift the soft palate back up and release the tongue down.
To prepare for some of the movements you need to make in your mouth, you want to be able to move your tongue to touch your soft palate and then go back down and rest in your mouth. To feel how the back of your tongue raises to meet your soft palate and then moves back down, do the following:
Move the soft palate up and down.
Stand in front of the mirror and shine a flashlight at the back of your mouth. Locate your soft palate and try to move it. If you aren’t sure how to move it, say “Hung-ah” and watch the soft palate and tongue meet and then separate. Say the “Hung-ah” several more times until you can feel the muscles that move the soft palate.
Try to move the soft palate up and then release. If it still won’t move, yawn. The soft palate moves up at the beginning of a yawn. The tongue usually releases down at the beginning of a yawn. You don’t want the tongue to push down as it does toward the end of a yawn; you want the tongue to release down as the soft palate moves up.
Lift your soft palate and keep it up for four counts.
Review the preceding step to figure out how to lift your soft palate. When you can execute that motion, lift the soft palate and practice keeping it up for four counts. Release it and then hold it up again for four counts.
Holding up the soft palate is what you want to do when you sing or speak. Keeping the soft palate up keeps the resonance just right, because dropping the soft palate too low makes the sound nasal.
Move the tongue up and down.
Say the “Hung-ah” again and watch the back of your tongue move up. See whether you can move the back of the tongue up and down, as you do when you’re repeatedly saying a K sound. Practice moving the back of the tongue up and down until you’re confident that you know how to move it up and then release it.
Separate the tongue and the soft palate.
Lift the soft palate and release the tongue. Don’t press the tongue down, but release it so there’s space in the mouth or distance between the soft palate and the tongue. This opening is what you want to feel when you inhale and when you sing.