How do we get from Venice to Florence airport via train?
Train from Venice to Florence, then taxi from Florence train station to the airport. Buses are sometimes difficult when you have your baggage with you.
I agree with Kent that the taxi to the airport is best. About 25Euro to the airport. The taxi is quick. There is a bus that runs from the Pisa Airport to the Florence Airport with one stop at the Florence SMN train station. CLICK HERE to see the bus schedule. The bus is less expensive but the schedule may not be as convenient as you need.
You can also take the volainbus between the airport and the main train station; see http://www.ataf.net/Template/DetailLinee.aspx?idC=180&LN=en-US&Linea=Volainbus I did this last summer, it was easy. The bus station is about 200 yards from the train station.