We did the Heart of Tuscany drive a few years ago.
I am the navigator ( and a good one), husband drives. We found it quite easy, the signage is very good in that area.
We always bring our own GPS (we have a Garmin Nuvi—using smartphone for a map app will likely suck up all your data) and I am a “paper map” person so I bring a good detailed Michelin map along as well.
I spent a lot of time looking at the route, figuring drive times, etc before we left so I had some familiarity with where we were driving. I used Google maps and Via Michelin. You can “drive" the route by using street view.
I think I also tried to map it out and save ahead of time but it just would not work!
I found plugging in the next town on the route as we drove to work quite well. That’s pretty much what the signage will help you do- just look for signs pointing to your “next” town or village.
We may have gotten lost a little bit probably because I was not paying attention but when that happens- just pull over- reset your GPS and go. We did try to stop a lot so my husband could take his eye off the road and enjoy the views but I don’t really recall the roads being heavily travelled at all.
Anyway- that's what we have done driving in Italy, Germany and France and we’ve always had good driving experiences/good luck. Believe me if Italy had not gone so well I never would have been able to get husband to drive in Germany or France- now it’s not an issue at all.