In general American, pay attention to the three oral postures: the lips, the jaw and the tongue. They are all pretty relaxed. The tongue is in the middle of the mouth and dropped down behind the lower teeth.
You can check you are in the correct general American if you relax your lips, jaw, and tongue and sigh “uh”. Then you get the schwa sound, symbolised by ə. ə is in the middle of your mouth as in love, glov e, above.
If you forget the general American posture, do the də (duh) test.