Better to get a real stepper. As components from old (really old) HDDs, they are cheap.
yup and ...
also printers, photocopiers and similar electronics are an awesome source of stepper motors
I tore down my last printer and was surprised not a stepper in the set - almost all were DC with optical encoders for positioning feedback... which begs the question to the OP ... why do you want a stepper?
SparkFun page seems to be pretty good reference for basic understanding.
A DC servo motor is better than a stepper motor for most applications. Only low power, minimum complexity systems use stepper motors. The easiest way to convert a DC servo motor to behave like a stepper motor would be to replace the control circuit, not modify the motor.
I would mount a quadrature encoder on the DC motor shaft. Next I would pass those encoder signals to a microcontroller so it could keep track of where the motor is now. That microcontroller would also receive the phased control signals so it could know where a stepper motor should now be. By comparing those actual and wanted positions the microcontroller would drive an H-bridge to control the DC motor and bring it to zero error.
The advantages of that DC servo motor system is that it will fight big loads without missing steps. It will also only draw sufficient power to maintain tracking.
The controller is best programmed as a state machine that is indexed by the two incoming pairs of phase signals. It then checks the two next critical phase signals so is not confused by encoder vibration noise.