Milan Airport to Milan Duomo by train

Take the Milano Express train to the Centrale train station. Walk across the plaza to the metro station and take the subway to the Duomo stop. Walk out of the under ground and you should be looking at the Duomo.

Also the train from MXP to Milano Cadorna is good; the metro links Cadorna to Duomo as well. It should be a few minutes faster than the Centrale itinerary. You can just hop on the first train leaving from MXP, no matter if to Centrale or Cadorna.

Lachera is correct. We tend to stay in the Centrale area so that is my first thought.

Malpensa express to Cadorna, then the red metro line to Duomo. Super easy.

Alternatively, Malpensa express to Milano centrale and yellow metro line to Duomo. But I find Cadorna easier to navigate