There are two trails from Población de Campos. Both bring you to Villalcázar de Sirga.
Route 1: Stay on to the right through Población, keeping on the North side of the river and passing through Villovieco. In Villovieco you cross the bridge and turn to the right and follow the river (or carry on straight and join route 2 in Vilarmenteros, which has been paralleling this one along the road). If you turn right, the waymarking is hard to see but you can hardly get lost as the trail stays adjacent to the river until you get to a crossroad (P-981) and a weathered ermita. Turn Left on the road and into Villacázar de Sirga.
Route 2. Follow the road out of Población, over the bridge. The camino follows the road, through Revenga de Campos and Villarmenteros (both with bars and fountains) and continues all the way to Villalcázar de Sirga.