Front of the tongue at the "N" spot, the ridge just behind the front teeth.
Tongue fully spread along the palate but not pressing outwards on teeth.
Back of the tongue 'sucked into the throat' which will cause the head to want to move and rebalance, I find.
This way, the whole of the tongue is up there... and now to make it want to stay, simply SWALLOW.
To swallow, we are doing the "Mew Push Swallow" which is pushing the tongue up and swallowing that way, not activating any other muscles. In the mirror, only your throat will move if this is done correctly.
Swallowing properly with lips closed will create a light suction suction effect in the mouth. This is to be maintained, as a constant negative pressure inside the mouth. Further swallowing may increase this negative pressure.