1.start checking timetables-preferably not via app of Eurail. The national sites are best-but, this is ESpana-it may not yet be on for that date-too far advance. To get global idea-use sameday of week next week Site of bahn.com is best for general planning anywhere in EUR

2. now the hard part: this is ES; ALL trenes need also a RES and ES is #1 in making that dificulte muy: have to visit counter when there. There is also a tiny one in Barajas Aeropuerto T4. Show pass with activated passday-they will understand latino ES and arrange it. First to Barca, then onward
3. FROM Barca you can choose between hi-speed=RES and lot extra, plus by now still unknown connections. OR use RES-free trenes regionales, change at border to Francia, in FR also local TER trenes=slower, more changes, but no extra costo or worries.
4. Frankly: think again if a pass is really the best thing for thisiif you only want visit Toscana short time: search for cheap flites to Firenze or Bologna and then its just short hops. The way overland en tren will cost you 2 traveldays each way=leave nothing for local trips and this is only worthwhile if you also want to see, as you pass by anyway, some of those places, mostly in the FRancia along Mediterranee. There are also NO trenes de noche to cut off time along here. There will be BUSes de noche, like Barca-Marsella or Nice. You have to book and pay these and in general they may even cost same or less as the supplmt to pay for trenes AV=alta velocidad
5. Buenes viajes, retorno with more questions, but same-same has been posted here several times now last week. Also many tips on seat61.com