This sound is used in words like waar (where), laat (late) and garen (thread/yarn).
Start saying the British English sound ‘a’ (as in ‘car’). Drop your jaw even further, until your mouth is really open. This is essential to be able to pronounce the sound correctly. Your tongue is also lowered automatically. Then pull your cheeks apart (as if you smile a little). You should really strain your facial muscles. It is a relatively short sound, but not too short.
Now you should be able to pronounce the sound aa.