First light without comments and red.

#include "FastLED.h"

#define NUM_LEDS 20

#define DATA_PIN 12
#define CLOCK_PIN 11

CRGB leds[NUM_LEDS];

void setup() {
    delay(2000);
    FastLED.addLeds<APA102, DATA_PIN, CLOCK_PIN, BGR>(leds, NUM_LEDS);
}

void loop() {
   for(int redLed = 0; redLed < NUM_LEDS; redLed = redLed + 1) {
      leds[redLed] = CRGB::Red;
      FastLED.show();
      delay(100);
      leds[redLed] = CRGB::Black;
   }
}