Hello, I am looking to book an itinerary with multiple flights to/from Europe on Qantas & Partners. How can I get them into one booking? Can I only do that via the multi city option or can I somehow add the trips one by one? Thank you very much for your help.
The multi-City tool is your best bet. Alternatively you can phone up and get an agent to help you with the booking. In both cases though you will end up with a single booking with all the flights/sectors included.
This post has some good tips:
Qantas now allows you to search up to 12 flights - but you can “trick” the system by following the tips in the linked post to “skip” explicitly required stopovers etc…
Like the old adage says “a bird in the hand…” - so grab flights when you see them. If you can’t get all your flights on a single PNR/Itinerary, the next best thing is to have all outbound and then all inbound flights on their own bookings. Then (for the most part) baggage is taken care of in terms of being booked through etc… This also helps to mitigate the scarcity of reward seat availability.
Once you have a booking with a few flights or more, you’ll need to call up Qantas to make additions to the itinerary. That will cost you points each time. So be aware of that. Best to try and minimise those somewhat tedious phone calls. 
But you pretty much just rinse and repeat this process as you build your desired itinerary. Best of luck. 
Thank you so much