You’ll need a minimum of 420,000 KrisFlyer miles for 2 return business class flights from Sydney to Rome. That’s for a “Saver” booking. “Advantage” redemptions are more readily available, but require more points (in your case, you’ll need 600,000 points).
Singapore Airlines opens up its bookings 355 days in advance. The sooner you book, the better, and the more likely you’ll be to snaffle a “Saver” redemption.
It’s easiest to book online. Go to the Singapore Airlines website and click on “Redeem Flights”. Alternatively the Singapore Airlines app is very good. If you book using the app, select “KrisFlyer Redemption” and go from there.
If you don’t quite have the number of KrisFlyer miles to get to Rome, flying to Istanbul will set you back a mere 312,000 miles (Saver).
Enjoy your trip!