Hello, I am a newcomer in this site so you can see I got no reputation at all. But, as a professional electrical engineer. I can tell you what needs to be done to complete this work.
1. We will create the code for the stepper motor. This could be as simple as generating PWM from arduino to be converted into angle, not to mention all those libraries that are helpful for stepper code.
2. The rocker switch will be connected with 2 digital pins and used as a logic gate to choose which side of the slider that needs to be controlled.
3 Potentiometer is related to analogWrite code as it is used to control the speed from the stepper.
I'll be glad if I can write the entire code for you.
Regardrs.