I don't know if it's the best way, but it has worked for me: I've always learned Italian as an autodidact. I started with the Assimil method, based on reading and listening in such a way that you end up memorizing some sentences with the basic structures of Italian.
Then I used the site http://www.busuu.com to do writing exercises (I didn't find very useful the other kind of exercises proposed by this site). It works as a social network of people interested in learning languages. The site proposes you writing exercises that you can submit so that you'll get a correction from a native speaker. Each user of the network is both a student of a foreign language and a "teacher" of its own mother tongue. The interesting fact is that you can make your own circle of reliable Italian friends so as to send them your own written exercises. I'm not sure if this resource is nowadays as good as it was in the past because I haven't been using it very much lately and some people told me that it has changed in worse.
At the same time, I got an Italian grammar book for foreigners that I consulted whenever I had doubts, but not to do grammar exercises. I tried instead to use the language by doing these writing exercises and by communicating with my Italian Busuu friends. And I tried to read a lot (real Italian literature, I hate adapted books for foreigners) and watch some Italian films.