What I suggest is:
_Wake up and make brekfast at COVA (Via Montenapoleone). It is an institution. You'll findo that in many cafes breakfast COULD seem god but croissants quality in Milan had dropped to death. Cova's still amazing. Cappuccino there is also very good: creamy and not boiling hot like those ones at Starbucks.
_Since you're in Montenapoleone, you'll find brands that are available all around the world (Gucci, Etro ecc) and other that aren't. You're not going to spend a lot of time here, besides getting yourself in the Milanese Atmosphere. What I really suggest to see is: Lorenzi (in front of Cova) in which you will find lots of interesting products for shaving, hair and so on and so forth. Larusmiani Tanino Crisci Sabbadini. Then, you have Rubinacci of course; and another very talented tailor who's atelier is in Via del Gesù. His name is Tindaro de Luca and he's at the same level of Rubinacci for me. You move around Monetapoleone a bit and you find a great shop nearby, Castellani in Piazza Filippo Meda (5 minutes from Montenapoleone). There is also Eddy Monetti
in Piazza San Babila which is very good and Saintandrews Showroom in Via Bagutta 8.
If you're hungry, you can stop and eat at a very good restaurant in Via Montenapoleone12 which is called Il Salumaio. If you're lucky (or unlucky, it depends if you enjoy the company) you may find Lapo Elkann there, Luca Rubinacci or Mariano, and other people like this.
What about a nice shirt? You can move to Camiceria Siniscalchi, in via Carlo Porta. Beaoutiful shirt. While reaching via Carlo Porta, stop by A. Caraceni (the real one, togheter with Tommy and Giulio in Rome, the others are "seconds") in Via Fatebenefratelli 16. You can spend all three days in Caraceni, or in camiceria Siniscalchi taliking with Mr. Alessandro.
Then (or if you don't want to go that way) you should absolutely walk into via Torino and find a little street called via Soncino: a very surprise because via Soncino is like 20 meters and you have a bespoke shirtmaker, Camiceria Ambrosiana and a bespoke shoemaker (friend of mine) Antonio Pio Mele. Pay a visit to both because it's worth it, in my humble opinion.
But to make a suit or a shirt you need fabrics, right? Fort shirting, you can go to Clerici in Via Dante, or Spadari in Via Spadari. You'll find yourself passing in front of the worldwinde famous and expensive Peck Gastronomy.
Wait, you said you needed suiting fabrics? There is Guenzati in via Dante: you'll easyli find coverts over 1 kg/meter and a rainbow of Harris Tweeds and many other fabrics.
Or, if you prefer (I prefer!) Il Vecchio Drappiere in Via Meravigli. Those shops are run by the same familys from generations.
While in via Meravigli, get a coffe in one of the oldest and best cafès in Milan: Marchesi. In Dicember, they make the best Panettone in my opinion.
And now? There is a fork in via Meravigli: if you go straing you go to Marchesi Cafè, if you go left there is Marinella Ties, if you go righ you find a lot of shops also: starting from another Lorenzi shop, you'll find M. Bardelli. Three floors of made to measure tradition. You'll be like a child into a toys game there.
Well, this is what came to my mind at first. I hope it's enough; if you like or have more time, you'll get more.
Enjoy my city.