The best way is probably this: get born from Italian mother and father who also teach Italian at school.
The second option would be open to a high degree of varieties, especially due
to personal interests and tastes, and the good things that life is preparing for each of us.
The more your learning path matches your personal interests the fastest you will be learning, this is due to the mechanics of our mind when selecting what will go to permanent memory and what will not go there,
and emotions (i.e. doing something you really enjoy) has a key part in this process.
For instance I just LOVE reading, so for me one of the best ways would be (after learning some basic grammar and a bunch of words), to go through a bilingual text.
I also love songs, so much of the English I know comes from songs of Leonard Cohen, Bob Dylan, Nick Cave, Johnny Cash and many others.
I am telling about myself as I don't have any clue about my interests, so I would humbly suggest you to make a question similar to this, but stating your interests, somebody might come up
with good suggestions.