We actually did this in reverse on an outing from La Paz but this would have been a great route immediately upon crossing the border.
Crossing the border at Desaguadero, take Ruta 1 east until you reach the town of Tiwanaku after about 44 km. The ruins are divided into two groupings with a small area on the southwest side and another larger grouping on the east side of the small town.
After visiting the archaeological site, leave Tiwanaku along the northern route. This will take you on a route that traces a pointing finger into over a stretch of land that protrudes into Lake Titicaca. It’s all dirt and pleasant landscapes eventually returning to Ruta 1 at Tambillo about 15-20 km east of Tiwanaku. From Tambillo it’s a pretty straight shot into La Paz. But leave yourself time because navigating La Paz is more challenging than this route!