You can create an account on eurail.com and register your paper pass. Most seat reservations are available but 2€ booking fee per person is added.

Otherwise have a look at these 2 links :

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

https://www.seat61.com/interrail-and-eurail-reservations.htm

Pass and seat reservations are totally separate so which one you have doesn't matter.

For further questions mention route and travel date and we'll gladly help.