I found this explanation, given by Mister Micawber, very helpful
"It is not the tongue, it is the tension of the apparatus and the form of the mouth. For the /æ/ of 'bat', smile widely and tense your cheeks where the smile ends. For the /ə/ of 'but', relax the jaw and mouth completely and let the mouth drop open only enough to let air pass through, then simply voice an exhalation"
Could you give the explanation of other sounds like the o of 'word', the i of 'bit', the u of 'cup' and the s of 'measure'?
Thank you in advance.