You need to come to Pisa Train Station to catch the train as specified in the train tickets you will receive by email after your booking.
After you arrive to Florence train station, you need to exit and head towards the meeting point in Florence which is about 5 minutes walking from the station. Here is the meeting point information:
At 3:30 PM - In front of Tourist Info Point - Piazza della Stazione 4 –Florence (View on Google maps)
Pisa Central Train Station,
Piazza della Stazione, 56125 Pisa PI, Italy
The exact time of the train departure time will be sent to you by email.