I'm guessing you scroll the selector wheel to "Trim", then pick which channel to adjust. Typically, you set both to 0 for throttle and steering. Then get it as close as you can to where "center" is for steering and "idle" for the throttle linkage mechanically by adjusting the collars on the throttle linkage and the turnbuckle for the steering. Then you can fine tune it using the trim on the radio.I'm not sure how to do the throttle trim on my controller??
About 7 minutes in, this shows you trims: