GA /ɑː/ is an open, back, unrounded vowel. The tongue tip and blade should lie as flat as possible in the mouth. Open your mouth wide (as if a doctor were holding down your tongue with a depressor and asking you to say ah) and let the back raise slightly.
The jaw for GA /ɑː/ and /æ/ tends to be more open than for the open vowels of AN. There is also more muscular tension in the lower jaw, in a way we find difficult to describe. It would appear that the muscles of the tongue and lips, as well as the muscles at the corners of the mouth and just under the chin, are tensed.