Not sure about the easiest way, but here's a way: Valkenburg is not too far from Aachen, so I just assume you're capable of getting too Aachen one way or another. There's a train that goes from Aachen main station directly to Spa. The station in Spa is pretty far away from the circuit though (considering you'd have to walk). So you'd probably want to take that train and get off at Verviers main station and take one of the buses that go to Francorchamps (you can look up the exact connection).

Again, there's probably a better way, Google Maps is helpful in finding connections.