If your flight got cancelled because of the Qantas grounding and you haven't yet received an email from Qantas confirming that you'll get a free return domestic flight as compensation, there's now a form on the Qantas site that lets you register for the offer.
Note that the form only covers domestic flights; international customers are going to receive a separate offer, which hasn't been detailed yet.
I got caught out by the cancellation and I have already got my email, but I have to say Qantas is making this whole process rather convoluted. So far, I've been sent one email saying that the offer will be made, and a second email telling me specifically that I'm eligible, but I'll have to wait for a third email which should arrive just before November 30, which actually gives me the details I need to book a flight. And that's before considering the various terms and conditions. I'm not one to turn down compensation, but it does strike me that the approach doesn't entirely suggest efficiency.