I don't know if you intend to go to Montepulciano directly via the A1, or if you prefer to go via Siena and/or the Chianti (takes a bit longer but u might like it). But I'll send you toward the Firenze Impruneta A1 entrance, through the Via Cassia (SR2), then you decide whether to hop on the A1 or take the Raccordo toward Siena or the Via Cassia (SR2) all the way, or hit the Via Chiantigiana from the Via Cassia (through Ferrone).
Using Google Maps enter:
From: Hertz, Borgo Ognissanti....
To: Piazzale di Porta Romana.
Follow the yellow (actually blue) brick road on Google maps religiously. No errors allowed.
The ZTL camera is on the Lungarno Amerigo Vespucci just east of the bridge (Ponte Vespucci). You must turn right on the bridge just before the cameras (which are very visible and have a bright red street light). Once at Piazzale di Porta Romana you will follow Via Senese through Galluzzo to the freeways (autostrade).
Coming back, just set Google Maps the same way but in reverse. From Piazzale Di Porta Romana to Hertz. Then follow religiously. On the way back the ZTL camera is on Borgo San Frediano, just east of Via Sant'Onofrio. You must turn left toward the same bridge on via Sant'Onofrio. Once crossed the bridge go straight on via di Melegnano, then left on Borgo Ognissanti all the way to the multistory garage (across the street from the Hertz office). You can also arrange with Hertz a later return after hours if you wish (for a small fee).
Another thing to be aware. On the way back toward Piazzale Di Porta Romana from the freeway, Via Senese is bus lane only on the northbound direction between Via Del Gelsomino and Porta Romana (there is a Q8 gas station there). You must turn right on via Del Gelsomino and later left on viale Del Poggio Imperiale to get to Piazzale Di Porta Romana.
Go to Wikipedia, search the article on 'road signs in Italy' and become familiar with the international road signage system used in Europe.