From highroad A26 you have to take the exit to Romagnano Sesia and proceed to Varallo and Alagna Valsesia. At the end of the road there is a big parking (gps 45.865882, 7.936448) where you have to park your car.
From here you can follow the road along the river Sesia. First you will see Cascata dell’Acqua Bianca and shortly after you will reach Cascata del Sesia. This walk takes about 30 minutes (2 kilometer).
It is also possible to buy a ticket for the bus (euro 2) that brings you a little bit further into the valley to a parking near Cascata dell’Acqua Bianca. I think outside the high season you can drive to this parking.
From here you also can walk further into the valley (for 2,5 km) to alpe Bors/rifugio Crespi Calderini (alt 1.829 m) to see the tallest waterfall of the region, Cascate delle Pisse.
It was a long time ago I visited Cascata del Sesia and although it isn’t that big, the waterfall looked very nice and powerful.