https://medium.com/jungletronics/uln2003-28byj-48-stepper-motor-f1cc5357eff

_61_ULN2003_WAVE_DRIVE_01.ino

// WAVE DRIVE

#define stepper PORTB
#define waiting_time 500

void setup() {
  for (int z = 8; z < 12; z++) {
    
    pinMode(z, OUTPUT);          // pins 8,9,10,11 output for driving the motor
                             
  }
}

void loop() {                    // WAVE DRIVE - one coil at a time
  stepper = 0B0001;              // 0x01;

  delay(waiting_time);

  stepper = 0B0010;              // 0x02;

  delay(waiting_time);

  stepper = 0B0100;              // 0x04;

  delay(waiting_time);

  stepper = 0B1000;              // 0x08;

  delay(waiting_time);
}