Timetables for August aren't published yet. You need to wait.

There's no need to validate anything for a specific train at the station. The railpass is your ticket; you just need to add the trains in the app.

To make sure I understood, I need to keep checking Rail Europe, once trains are available to book, I book it providing my Eurail Gobal pass number. After that I add the train details in the Rail Planner app. Did I get that right? Any other steps I need to do?

Thanks again for your valuable input. 
 

That's totally correct. 

But there's no need to hurry. There's no quota for passholder reservations in Italy and the trains won't sell out that fast. 

Just pay attention when booking. There are some trains going very fast (about 90 minutes) because they do avoid the stop in Napoli Centrale; might be the best ones for a day trip.

Thank you. That was helpful. Is it easy to cancel/change reservations after booking? 
 

Some reservations can be exchanged for free/for a fee. Have a look here (same thing for Eurail) : https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy/exchange-and-refund-conditions-for-reservations

Simply make them on the day/a few days before when your plans are a bit more definite.