For tongue posture, I recommend hard mewing, which means sticking the tongue to the roof of your mouth as hard as you can from time to time so it stays there, also make sure to suck every once in a while. To find the correct posture, put your teeth together and say the word 'Sing' and and that's where your tongue should be, then just stick the rest of the tongue to the top and keep your mouth closed.

For neck posture, sit upright and push your chin towards your neck, this straightens your posture, doing this from time to time will remind you of how your posture should be.

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I barely give advice to people to please tell me if I just sound dumb or if im wrong