Installing a pot to control a Stepper Motor in Marlin firmware

Thanks for the info. I wish I could write a sketch but I am total, total lost. I have no idea as to where I would even start. If I was into doing this about 50+ years ago I might be able to do it, but mother time has gotten the best of me. Thanks for responding. I was hoping someone might have what I need and I could just plug and play, so to speak.
thanks again.
RC