From Zurich drive to Constance (N1 and N7) and take the ferry to Meersburg. Then drive in direction of Friedrichshafen. After a winding road heading for Salem, Heiligenberg, follow the signs to Pfullendorf. On the road to Pfullendorf turn off in Rickertsreute in direction of Illmensee.
Follow the B30 until you reach Biberach. Then go in direction of Saulgau over Ostrach to Denkingen. In Denkingen turn off to Illmensee.
Follow the A81 to Stockach and from Stockach head in direction of Pfullendorf. Drive in direction of Ravensburg and after 12 km turn off to Illmensee.
Drive to Markdorf and then in direction of Deggenhausertal, Höchsten. From there follow the signs to Illmensee.
Alternatively you can choose the route over Ravensburg.
In Ravensburg head in direction of Meersburg (RV-Weststadt). After Ravensburg turn off to Ostrach-Pfullendorf and go to Wilhelmsdorf. In Wilhelmsdorf follow the signs to Illmensee.