From the Desenzano del Garda to Salo road (SS 572), turn onto the road sign posted to Lago Idro and Valle Trompia. The road dives into a tunnel and after some turns around a complex junction joins a partially built, new fast road running north along the valley (SS 237). You'll emerge onto the old road running through Vabono and other small villages in the valley before rejoining the new road, which is mostly in tunnels to avoid existing villages. In future, when the new road has been finished the driving will be easier, but the views limited as the road is mostly in tunnels.
If you have time you can actually take a very steep mountain road from Gargnano to Idro (SP 9) over the mountains via Lago Valvestino, which is prettier but much slower.
Near Barghe stay on the SS 237 (avoiding the turn that carries on to the Valle Trompia) and carry on climbing up a narrow valley to the mouth of Lago Idro, then through yet more tunnels high above the lack, before stopping at Anfo for a brief lakeside walk. (if you took the road over the mountains you eventually join the road at Idrio) At Anfo there's a cafe by the lake and some pleasant walking along the lakeshore. Take care to look for the castle (Rocca) high above the valley.
Carry on the SS 237, signed for Trento, before after the end of the lake turn right onto the SS 240 near Storo. The next section is very steep and requires snow chains in winter. Head up the steep zig zags, taking care of avoiding other traffic on the bends. The road climbs and turns in a series of sharp bends, before it levels out in a steep sided valley that looks typically alpine. Head down the road past a series of small mountain villages to descend to Lago di Ledro. Stop at the obvious car park, and cross the road, and climb up to the view point on the old road. Just think, that although the current road is twisty, the old road looks absolutely terrifying with ungaurded drops, narrow sections, and even sharper bends.
Descend to the tunnel, which is several Km long, but thankfully wide and easy driving. If you get a chance, look for the brief gap between tunnels where you can see down to Lake Garda.
Stop in Riva for a break and try walking along the pretty lakefront, to see the lake crowded with windsurfers and sailing boats, the huge bulk of the mountain above which you have just driven through; and at the bottom, where the sheer rock meets the lake, the openings of the tunnel you are about to drive.
Join the Gardesana road, SS 45 bis, and follow it south along Lago di Garda (sign posted for Salò). After the massive hydro power station the road soon plunges into a endless series of tunnels below the mountain. The longer tunnels are well lit, but the shorter ones are not, and often have bends and very narrow stretches with no line down the middle to guide you. Combine that with heavy traffic at times, and this is a drive that will have you gripping the steering wheel very tightly indeed. This is where during the filming of the Quantum of Solace Bond filem a stunt driver managed to put a car in the lake, near Limone sul Garda. If time permits, you can escape the main road to visit some of the small villages along the lake shore.
As well as the deeper and longer tunnels, you'll see a very distinctive type of tunnel, where the side facing the lake is partially open, with either big pillars or frequent side tunnels cut in the rock. You can, if you can take your eyes off the road long enough, look out across the lake to the far shore and Monte Baldo. You'll also get brief sections outside the tunnel where you can see ahead down the lake, and see small houses and hotels built into the mountainside. There are some very limited places you can park to stop and view the scenary, but getting into and out of the parking spaces can be quite tricky.
After a while the tunnels become shorter and lesss frequent, with the last tunnel before Gargnono. The road then climbs above a series of small villages, where the views of the lake are blocked by buildings. After this, the road drops down to the shore with a series of pretty villages, and then grand hotels and gardens hinting at a very glamourous past for the Gardone Riviera between Gargnona and Salò. Sadly traffic along here can be very heavy, and although the road is less tricky to drive, you'll find traffic slows you down.
Near Salò, take the road sign posted for Bescia which passes above Salò, offering some lovely views over Salò and the lake, before taking you back to the start. If you have time you can go via Salò, but be warned the traffic can be heavy in the town, even if the lakeside promenade is very pleasant to wander along.