The first phase of the run is the set up, then you wait then the second phase is the execution.
During the set up you need to form Italy, obviously, but you also want to build good relations with as many nations as possible. You’ll want to build lots of ships, so you have more interests - and can place them all round the world. You won’t want to expand too much so you limit your infamy and stop others seeing you as a rival - I took my provinces back from Austria (and then started buttering them up so they dropped the rivalry), colonised Oceania and a bit of Africa and (just so I had something to do) conquered my way around North Africa vassalising then annexing Morocco through to Libya and sniping Sinai and the Greek islands. Ideally, you’ll want to stop Germany from unifying (and any other major expansion) so there are lots of countries to fight each other and ally with.
Then you need to wait, not just till you have multilateral alliances tech, but so do most of your potential allies. For me this was around 1885. Once you’ve finished exams don’t spend your time building relations up to +50 with as many countries as physically possible.
On to execution: you need to watch every diplo play around the world. What you want is a play with two supporters on one side who you have amicable relations with. Then join that side of the play. Wait for a new week and you will get a big boost to relations. You should then be able to ally with both the supporters, not with the leading nation. Then switch to the other side of the play and the achievement will automatically pop.
