A direct fast train to Florence is 2 hours. Standard base price is €112 each. Buy in advance either through the trenitalia app or online through a browser. There is no customs to worry about at Malpensa, or Linate, unless has something to declare. it’s immigration. It’s all automated machines. You exit the plane, enter the terminal, and the officials will show you where to go. The machines take your photo after scanning your passport and then afterwards you walk up to have your passport stamped. When we arrived on Saturday at Malpensa they were separating those of us with USA passports from everybody else. We were out of the airport in less than a half hour. Afterwards there are signs how to get to the train which is per of the airport services. It’s about a ten minute walk when the automated walkways are functioning otherwise it’s 15 minutes. Tickets for the Malpensa express can be purchased using machine or agent both of which are next to each other above the train platforms. It’s €13 each. The train to Milano Centrale is about 45 minutes. After which you look at the timetables to see what platform your train to Florence will be. Easy peasy. The Malpensa website has a ton of info on this. You can purchase both tickets at the same time on the Trenitalia website as well but you’ll still have to change in Milan. When searching their schedules you’ll have to enter the place names in Italian. For example, Milano for Milan, Firenze for Florence, other wise it will give you errors and won’t know what you want. Good luck and happy travels.