[Alan Anderson]

A servo is a tiny motor with a built-in position sensor, along with a small circuit that controls the motor in order to make the sensed position match the commanded position. You will need to duplicate that sort of system with a motor from the Kit of Parts and a position-sensing device such as a potentiometer, along with programming that controls the motor in order to make the sensed position match the commanded position. A simple algorithm would be to find the error between the desired and measured positions and drive the motor in the appropriate direction until the error is near zero.

For a fancier and more effective algorithm, search the forums for PID position control.