In order to arrive in Barga, you should first of all reach Lucca, easily accessed from the Florence-Pisa highway (aka Firenze-mare).
Once past the pay-toll you will turn left (Viale Europa), continue on the bridge over the railroad, and proceed straight on Lucca’s ring road (keeping the city ramparts on your right). You will follow Viale Papi, Viale del Prete, pass Piazzale Martiri della Libertà, and finally hit Viale Batoni. On Viale Batoni keep the middle or left lane in order to turn left after the traffic light on via Civitali. On Via Civitali proceed straight after the first traffic light and then bear right at the end of the road on Via del Brennero
The rest will be quite easy.
At the end of this long stretch of road (approx 7 km) which coasts the river Serchio (even though you don’t see it), before Ponte a Moriano, it is advisable to turn left on the bridge (the junction is well marked). Follow the road past the town of Borgo a Mozzano. The first bridge across the river will take you back on the right bank of the river, which is also the right one for Barga. Before the town of Fornaci di Barga you will see indications for Barga, which at this point will be 4 km away.
In town everyone knows Casa Cordati. The exact address, anyway, is: Via di Mezzo 17: a few dozen meters after entering the Porta Reale, which is the first door you will see arriving to town.
Have a safe trip.