For the A vowel, your lips are neutral, released and relaxed but the front of your tongue is slightly raised. The danger with this vowel is that here in the U.K. it can become dulled or blunted - so that A in “cat” sounds closer to “U” in “cut”. Other accents - for example the German accent - can push this sound towards “E” as in “get”. So “cat” might sound like “ket”.
“A” is a sound that CAN be safely lengthened - and this can create quite an effective emphasis as in the phrase “the seven ages of MAN”, for example.
When the A sound is correct, it is often described as sounding nice and “bright”. Keep this idea of a bright sound as we work through the exercises.