You may find it helpful to have a look at this Thread on the same topic.....
https://community.ricksteves.com/travel-forum/italy/traveling-from-venice-to-cinque-terre-e0793aef-7a1c-4861-92c4-4b7bd8c289f0
The trip from Venice to the Cinque Terre is very straight forward, and as I mentioned in the other thread, the route through Florence is usually a bit faster. Note that you'll have to use the correct spelling in Italian when you purchase tickets. You'll be departing from Venezia Santa Lucia and arriving at Monterosso (the name is actually Monterosso al Mare, but abbreviated to Monterosso on the Trenitalia website).
Depending on what time you need to return to Milan for your flight home, you may be able to use one of the direct trains from Monterosso to Milano Centrale (there are a few of those each day). It's a very easy trip, but takes a few hours.
If you're willing to commit to a specific departure time, you can pre-purchase tickets for the faster trains (Freccia, InterCity) at www.capitainetrain.com (very user friendly website and the same price as buying through Trenitalia). If any segments of your trip will be via Regionale trains just buy tickets for those when in Italy, either at a Kiosk or staffed ticket office.
If you have any further questions, post another note.
hubestur,
" For some reason, that's a really big deal that for the life of me, I don't completely understand."
A bit of clarification is in order. ONLY tickets for Regionale trains (and also Metro, Bus, etc.) need to be validated (time & date stamped). As a previous reply indicated, tickets for Regionale trains are sold as "open" tickets which can be used on any date within a specified period of time. Again as stated, people could simply continue to use these day-after-day and only pay one fare. When tickets are validated, they're only usable for a set period of time, usually about six hours so they can't be re-used. Therefore it's critical not to try and save time or whatever and validate these the day prior to travel, as by the time they're used they will have expired. The important point to note - those caught with unvalidated tickets may be fined on the spot, and fines start at about €50 PP. THIS PHOTO provides a clear illustration of the rules (this photo was taken on the Leonardo Express but the rules are the same for other trains).
Tickets for the faster trains such as the Freccia or InterCity trains do not have to be validated, as the tickets are specific to ONE train, date and departure time, and can only be used on the ONE train and date specified on the ticket. If you were to board the wrong train by mistake, you'd be deemed to be travelling without valid reservations and (you guessed it), subject to fines on the spot. With the fast trains, the fines are the same but they also charge the cost of the reservation, which adds about €10 PP (for Freccia trains).