I started with the standard "warn the pope and venice" method and obtained Romagna, Treviso and later Rome this way. In the meantime, forged a core on Modena and unsuccessfully tried to forge a core on Urbino. I started warning more Italian countries and allied Milan, waiting to get involved in a war against Modena or Siena. This happened at some point, and I annexed Siena, long-waited Modena and vassalized the country. This was ca. 1500. Then peace until ca. 1530 when Venice declared war on someone again and from this war I got Verona and soon after diploannexed Modena (in Ferrara) with autonomy statute, leading to a fairly large connected territory.
But I didn't start the game with the goal to unify Italy asap and also had taken another trade NI and switched to a merchant Republic to get the Specialized Traders bonus, so I wasn't in a hurry to get many more provinces. Instead, I snatched Alexandria from the declining Mamluks empire and also took a Naval NI. But I did (try to) start forging cores on Urbino (unsuccessfully) and Parma (eventually with success). Of course the Reformation also tends to slow down expansion. The next step was actually getting Sicily and Sardinia (with long wait) in the course of a long and involved war with Spain which did not last much shorter than it could have (ca. 1565) given eventually very low manpower and rebel problems. In 1580 I still didn't own any more Italian provinces, although it had been easy to get a few more provinces in Egypt including Cairo. During 1580-1600 the Ottoman Empire had lots of rebel problems and somehow accumulated extremely high war exhaustion (which wasn't my doing at all), resulting in some breakaway states like Croatia, Serbia and large Bulgaria (Austria had not been that active in the Balkans for a change) and even more wars for them, so I took the opportunity and jumped on the bandwagon to get some provinces in now relatively badly-defended Greece. Then a bit later the religious war started (in which Tuscany was leader of the Protestants), which I eventually used as the opportunity to dissolve the HRE (which was only possible because Austria somehow didn't participate and a relatively weak Bohemia was HRE) and that was also a very long war producing lots of war exhaustion and BB, so further expansion in Italy still took some time to take off after that, although I did vassalize Milan in that war.
Thankfully, Austria and France showed little interest in intervening in Italy during the next stage and the next targets to annex were Milan and Mantua, while Savoy was force-vassalized and thanks to my core in Treviso I could eventually forge a core on Venice. But I didn't rush these things and it took until 1645 to consolidate all of Northern Italy minus Savoy (vassal), the Venice province, and Austrian Friuli and Trent, after long-waiting Genoa. Instead, Tuscan Greece and Asia Minor was also expanded, which was almost too easy with periodic Ottoman bankrupties.
1650-1710 was maybe the most intense (and fun) period of the game with the wars against French-Austrian alliance (Savoy was an excellent buffer-vassal btw) and also saw the vassalization and annexation of Naples, the annexation of Venice, vassalization of Switzerland, more conquests in the East (Ottomans had recovered somewhat, but by now had become too weak to pose a great threat) and beginning territorial gains against Autria and France (in the beginning, I had to fight bitterly to get a white peace, and that slowly changed). This was also aided by the fact that I got some pretty good rulers during that time.
So, it took until ca. 1700 for the unification of Italy to be nearing completion; Urbino funnily turned out to be the last province to be annexed and clung to its independence until ca. 1730.
As very often, the 18th century allowed for the most territorial gains and near-permanent growth after the foundations for that had been laid in previous centuries.
Nice coutry Urza. In which order did you attack the countries to unite italy?
I've found that by far the easiest country to unite Italy with is Milan, from 1399 start.
Start by attacking Sardinia (Genoa has an unfortunate tendency to ally them) and long-wait Liguria.
This shouldn't trigger any HRE sanctions yet.
Then you only need Rome and Naples captured and cored - and they are both outside HRE.
If you are lucky, you can form Italy before 1500.
Also, you don't need to relocate your capital to Rome when doing this.
Still playing the mod now and then.. In many ways still the best game (released game, heh) I know, and there always seem to be interesting goals remaining which I haven't tried pursuing yet. Such as this:
Alexander was much faster, though. Also, Greece exists, it is a kingdom and was formed by Morea (by decision even) during the 1500s.
Not so fun: The regularly (every 1-2 decades) happening Sunni rebellion event which together with the Hindu rebellion event spawns rebel armies in almost every province. (taking the time to convert the minorities does pay off here, sadly I didn't bother.)