How to be a prince, by Niccolò Machiavelli to his BFF Lorenzo de' Medici:
Step 1: get yourself a kingdom, and preferably have your own army while doing it since mercenaries are bad news. Be careful when choosing a place to take over. Even though it will be harder to conquer at first, choose the land of a king with no powerful barons or ministers, because it will be easiest to maintain in the long run. Make sure you kill anyone who might oppose you before continuing. And choose a role model.
Step 2: keep your kingdom secure by not allowing people as strong as you are into the neighborhood. Also, make friends with your neighbors. Don't let people hate you, but don't worry too much if they grumble a bit. Maintain a reputation for awesomeness. When in doubt, think of Cesare Borgia.
P.S. Pleasie weasie come rule Italy using the steps Machiavelli showed you. You can do it!
Okay, okay, we'll break it down a little more:
Chapters 1-4: States can be republics or kingdoms, old or new. The easiest to rule are old hereditary kingdoms, lands that are passed down from father to son (and rarely from father to daughter, or mother to son or—you get the picture). Basically, instead of passing along their 2001 Toyota Camry, your parents give you a kingdom.
You'd have to be an idiot to have problems ruling one of these. Because they're so easy to rule, they are hard to take. The opposite is true of states that are easy to take: they tend to be hard to rule. The best way to take old hereditary kingdoms is by killing the old monarchy. Every last one.
Chapters 5-7: You also need violence to take self-governed republics, because they will rebel if you don't crush them. Just remember not to keep being violent. Get it over with so you can start being nice and people won't hate you. Never let your people hate you. Lie, cheat, steal—just don't become hated.
And make sure you have your own army.
Chapters 10-14: Mercenaries and auxiliaries (people you pay to fight for you) are a waste of time and dangerous, to boot. If you have a strong army, and your people love you, no one can touch you. They won't even think about it.
On that topic, you need to run your army, so war needs to be on your mind all day every day. You need to be on the cutting edge of war techniques and technology. By the way, a word (or two) on fortresses: they look cool and everything, but they can also make people resent you. They're really only useful if you are afraid of your people.
Chapters 16-23: Throw parties for your people. Listen to your ministers (the smart ones) but avoid brown-noses.
Chapters 24-26: Finally, Italy is not doing so great right now because its rulers didn't follow Machiavelli's rules. They blame bad luck, but you can always prepare for luck, and they didn't. Don't be like them. Be awesome.