That piece of code doesn't help because it's calling additional functions that you didn't provide. However, FastSPI is a very simple library to work with. It first creates an array 'leds' that you can use to address the entire string. If your string has 20 pixels on it, you would've set NUM_LEDS to 20, which in turn sets that 'leds' array to the correct amount of indexes.
You can loop through the whole array to set a color, for example:
memset(leds, 0, NUM_LEDS * 3); // this resets all values to zero
for (int px = 0; px <= NUM_LEDS; px++) {
leds[px].r = 255;
leds[px].b = 0;
leds[px].g = 0;
}
FastSPI_LED.show();
That simply sets each pixel to the color red, then "shows" the result.
If you want to address a specific pixel, do that by passing it's index. For example, if you want to address the 10th pixel: