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Pushing sideways with the tongue.
Yes, gently and evenly apply pressure on the whole palate but do not touch the teeth if you can. Otherwise, depending on your age and case, you should consider to get a palatal ( probably skeletal) expander.
Don't get mad about "mewing", by the way . Don't think to much about it. It has to be natural , not forced.
This may help:
She says there shouldn't be air space in the mouth.
Basically if you correctly and entirely fill your mouth with the tongue, you SHOULDN'T BE ABLE TO MOUTHBREATH and the same time be able to breath through the nose