Funny you asked. As I am playing an AC Milan save right now. I wanted to restore them for glory.
First thing off the bat, don't expect to win the Series A in the first season. Juventus was invincible the first season for some reason, only lost 1 game all season and ran away with the title.
However don't be discouraged. I managed to seal a 3rd place finish and qualify for champions league play the next season.
Here's what you need to do right off the bat -- make sure you keep De Sciglio and El Shaarawy. These guys will be key players for you. Sign them to contract extensions if you have to but keep them with your club longterm.
You start off with a small transfer kitty, so sell off some dead weight right away.
These are the players I sold in season 1:
Keisuke Honda 7m
Nigel de Jong 6.5m
Ignazio Abate 6.25m
Giampaolo Pazzini 4.1m
Riccardo Montalivo 2.5m
Michael Essien 550k
Philip Mexes 425k
Daniele Bonera 190k
So that gave me roughly 30m more pounds to play with.
Keep in mind that Series A rule limits you to 2 transfers of non-EU nationals per year.
In practical terms this means that if you want to dip into the talent hotbeds of Brazil or Argentina, you can only get 2 of them so make them count!
I chose to acquire DC Alvarez Balanta for 7.25m from River and CM Lucas Romero for 7.75m from Velez. I'm into my 3rd season now and these are still 2 of the best players at my club.
Lucas Romero has won European Golden Boy twice, Best player in Series A 3 times, and World Player of the Year once.
Your experience may vary of course but those two are very solid building blocks.
I also added DC Vallejo for 675k, LB Sallahi for 500k, AMC Vlasic for 525k, and RB Peruzzi for 3.9m, and ST Belfodil for 3.2m.
Peruzzi is a very good backup player for the money. Belfodil didn't really do much for me. Once Torres' loan expired I went after and got a much better striker for the longterm in Richairo Zivkovic.
I tried to sign Munir but he chose to sign extension with Barcelona instead. Drat!
So I loaned Samper from Barcelona for a season, he played a treat. But after loan finished he wasn't interested in being bought and... signed extension with Barca instead. Drat again!
The key to success imo is pick up 2 good tactics. One for home games and one for away games.
And maybe have a 3rd one in your back pocket just in case neither of those are working.
Keep building up your talent season by season as Ac Milian will give you increasingly larger transfer budgets over the years. In fact in season 3, AC Milan was taken over by a consortium in my save. And at the beginning of season 4 they gave me a 60m transfer kitty to use!! *wow*
By season 2-3 in my save, Napoli got stronger and Juventus got weaker. No obvious reason for this. Maybe Juventus just got old? Anyways I managed to win Series A twice in those seasons and also won the Champions Cup in the 3rd season by beating Manchester United in the Finals.
Season 2 AC Milan 1st, Napoli 2nd, Juventus 3rd
Season 3 AC Milan 1st, Napoli 2nd, Juventus 3rd
If you have any more questions feel free to give me a shout on this thread.