For trains without mandatory reservations you simply hop on and find a seat (and add the journey to your trip, plenty of advice for how to do it in this forum).
For journeys with mandatory seat reservations, you either need to book them on eurail.com (booking fee) or other train companies websites (usually no or less booking fee).
Seat reservations are separated from the pass, do not activate your pass to book them (there's really no need to). You simply need to show both the pass and the seat reservation whenever a ticket inspector checks tickets.
Mandatory seat reservations in Italy (and many other countries) can be bought on tickets.oebb.at (no booking fee). Add Interrail/Eurail Global Pass as a discount and select one-way tickets.
- 10€ Frecciarossa trains
- 3€ Intercity trains
Maybe the date is not available yet as the Italian summer timetable hasn't been entirely published. Naples to Pisa is either done :
- Napoli Centrale - Roma Termini 10€ Frecciarossa
- Roma Termini - Pisa Centrale 3€ Intercity
OR
- Napoli Centrale - Firenze S.N.M. (Florence) 10€
- Firenze S.N.M - Pisa Centrale reservation-free
Both options take about 4h30, multiple trains per hour
Edit : There seems to be a direct IC (3€) between both cities if you prefer a more relaxed experience (1 hour longer)
- Napoli Centrale 07:31 - Pisa Centrale 12:59
Oh that makes sense! Thank you so much for your reply. Ive never been on a train before or done anything like Eurail so was a bit confused haha. Thank you ! :)
No worries happy to help.
Let us know if you've got any questions