Well there isn't a great deal to it, any half brain dead person could pick it up. I'm not sure how little WOWPOS differs at Big W, but there is the scanning (sale mode), then you press "enter" and that is the payment (tender mode). You will focus on two buttons "Cash" and "EFT".
Cash is simple, hit the button, and just press whatever the customer gives you, the first option will always be the exact amount. If I need to pay $30.04, and I give you a $50, simply hit the "$50" button and it will tell you how much change to give.
The same goes for odd amounts, you can "Key amount" and type in how much the customer gives you.
EFT is also simple, the first amount will be exact, then it goes generally by $10, $20, $50, $100 cash out. Key amount is the same, and can be used to 'round up'/'total' or cash out.