We are at the austin (austin station) where to take HK Shuttle Bus K1 from exactly?

Well, you can take the free shuttle bus line K1 to Kowloon Station from Austin Station. You can find the stop by following the English guiding signs.
BTW, you'd better to show the Airport Express (Single Journey, Same Day Return, Round Trip, Airport Express Travel Pass), and Airline ticket / boarding pass before getting onto the bus. Actually, you can walk to Kowloon Station within 10 minutes.
In addition, the Airport Express train is not free. You need to pay HKD 90 from Kowloon to HKIA.