Light only half of the LEDs How can I only let half of the LEDs light up,
for example, LED 1 on, LED 2 off, LED 3 on...and so on.

I did it this way
(this example fades also all LEDs on and off)

   int x = 1;
   for (int i = 0; i &gt; -1; i = i + x){
      leds[0] = CRGB(i,0,0);
      leds[1] = CRGB(0,i,0);
      leds[2] = CRGB(i,0,0);
      leds[3] = CRGB(0,i,0);
      Serial.println(i);
      FastLED.show();
      if (i &gt;= 55) x = -1;             // switch direction at peak
      delay(10);
   }


but that is not so nice if you have 100 LEDs or so.
I don't know much about neopixel but as a general coding practice if you want to do something for every other object, you use the modulus operator like so

if(i % 2 == 0)
{
  //i is even
  //off
}
else
{
  //i is odd
  //on
}


So just wrap your current for loop in another one and to the above with the iterating variable.