The easiest (and cheaper) itinerary will be via Germany and Switzerland. So you're planning to stop en-route ? Basel and Bern are 2 nice cities along the way.
Use bahn.com for planning.
Please note that ICE trains out of the Netherlands require seat reservations : 3€ through tickets.oebb.at
Optional seat reservations on IC/ICEs in Germany (recommended in summer) : 3€ per train on ÖBB or 4.90€ per journey on bahn.com. Otherwise you may have to stand.
Seat reservations are not a thing in Switzerland, don't waste money making them (usually impossible anyway).
There are also some mandatory seat reservations in Italy :
- 13€ Frecciarossa
- 3€ IC
Those are also best bought through ÖBB (no booking fee) : add Interrail as a discount and select one-way tickets.
@ thibcabe: thank you a lot for the information!
Grtz remy