I started using this tool just days ago so here's my tips:

* Change the search so you can do it for each segment ("Search For Flights" from memory) - it's not the default

* Focus on outgoing before even thinking about incoming. Just set the flight back to whatever - you'll be ignoring it to start with

* For the outgoing, firstly search from origin to destination for the date you want. e.g. MEL-LHR. Then look for the round circles! If you see the round circle in a solid orange block, you've got a hit and this is a possible flight. Scribble it down.

* The painful part! To search other dates, you can only look 1 day at a time. Click the >> button for 1 day forward, << button for 1 day back. Or alter the search for a totally different date.

* If the quick search (e.g. MEL-LHR) doesn't get a hit, nor any other dates you'd consider, it's time to research! Find other ways to get from MEL to LHR that aren't "default". e.g. MEL-LHR can go MEL-SIN-LHR, MEL-HKG-LHR, but there's many other ways, including via America or via another Australian city. You need to look up each component (e.g. MEL-SIN, SIN-LHR) and see if you can string an itinerary together. It's painful and requires a lot of patience.

Good luck!