I am going to guide you through the process of making a circuit that will allow you to control a servo by turning a dial. At the same time I will explain what is going on in the circuit and how it works.
A servo motor is controlled by sending a series of pulses to it. This is called "pulse width modulation." Depending on the length of the pulse it will turn to a specific angle.
A pulse must be sent to the servo every 20 milliseconds. The pulse length will vary from 0.5ms to 2ms. If it is 0.5ms the servo will turn as far as it can anti-clockwise. If it is 2ms it will turn as far as it can clockwise.
By using a pulse-generating circuit controlled by a potentiometer any pulse length between 0.5ms and 2.0ms can be sent to the servo. This allows us to move the servo arm to any position desired.