As long as you made the steering links the same length, disconnect the servo horn from the servo, power up your Tx, set the steering subtrim to zero, power up your car, and re-attach the servo horn. Due to the way the splines on the horn and servo line up, you might not be able to get the steering perfectly straight. You then have to use the subtrim to correct for the misalignment in the horn/servo connection. Then drive slowly on the track, where you might also need to adjust the trim so that the car rolls straight when coasting.