Step 1 : make sure your wife can tolerate your Milan habit (ignoring her when the game is on tv, spending money on merchandise, sudden mood swing depending on Milan's performance)
Step 2 : baby inside the womb can hear voices starting around 3 months into the pregnancy. So after your wife is pregnant, make sure she's sitting close by you whenever you watch Milan games on TV, so that the baby inside can hear the commentary or at least your praises for the team (I guess that explains why my baby has a big temper, when he was inside the womb Milan wasn't performing too good so the comments he heard from me were mostly curses). And before bed, stroke your wife's belly and sing very slowly : Milaaaaaan....milaaaannnn....sono conte...MILAN... (or whatever the real words are)
Step 3 : during the labor, make sure you wear a Milan jersey (I wore the 90/91 season's jersey). You see, inside the maternity ward, most of the colors are white (the doctor's and nurses' suit and the clean white painted walls). So although a newly born baby can only see blurry images, a very bright red color will atttract his vision.
Step 4 : his eyes can't still see properly until he's 2-3 months old. So again, a very bright color will attract him. Dominate his vision with red colors as soon as possible. Decorate his room, wear Milan jersey as much as possible whenever nearing him but be very prepared for the occasional puke or burb that babies usually do that will ruin your precious shirt.
Step 5 : buy a lot of football toys. Being a Milan fan is not much of a use if my son doesn't want to play soccer. So far it works for me.
Step 6 : try sit him watching Milan games. This is quite hard to do for me since Milan games are aired around 9PM in Indonesia, by which time my son is asleep. And wishing him to sit quitely with me for 5 minutes is a huge task, let alone 90 minutes
Do above steps and hopefully your son/daughter will turn into a bonafide Milan fans. You can of course improvise. But there's an unwritten first step that you need to accomplish yourself : get a girlfriend/boyfriend that's willing to enter a marriage with you