Malpensa Express train is located inside MXP, terminal 1, plenty of signs. I took it last week. I think it was about 12 euros, and took about 50 minutes. I used to take the bus and think it was a little cheaper, but I found it to be a real pain, especially when there is traffic, as it stops and goes along ugly industrial areas, highways, and outlying residential areas.

Just follow the signs to the train, go to the Malpensa Express window, and buy the ticket. Be sure to ask which binario, or track the train to Milano-Centrale is leaving from, because when you take the escalator down there will be several trains saying Milano, going to very different places in the city. In my jet lag I mistakenly got onto a train Milano-Cordona, and didn't realize it until just before the doors were about to close. Milano Centrale is the last stop, and you can certainly get a ticket to Florence from there.