Easiest: direct TGV, €35 reservation.

Cheapest: regional trains all the way. But in this direction you'll need to stay overnight somewhere or use a night train. No reservations.

Compromise: Regional trains until Perpignan or Nîmes and then a TGV (€10 or €20). Or AVE until Lyon (€10) and then regional trains.

Direct TGV : 35€, very popular so book in advance.

TGV to Montpellier/Perpignan : 10€ limited fare, 20€ until the train sells out. Continue with reservation-free regional trains via Portbou.

Night train to the Spanish border (Latour or Cerbère) : 20€ for a couchette. The following morning either use a second pass day or buy a ticket for 13€. Good option to save on accommodation!

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

https://www.seat61.com/how-to-use-a-eurail-pass.htm

The website is very complete.