Download the latest release of the Arduino IDE for your operating system. You may already have this.
In the 'Tools' menu navigate to 'Manage Libraries'. In the 'Filter your search' box type 'Fast LED'. You should get something like this:
Click on 'install', wait until it installs then from 'File' 'Examples' scroll down and 'Fast LED' should be down towards the bottom of the list. You can then select one of the Fast LED libraries for example 'ColorPalette'. Select this and the code opens in Arduino.
Among the first few lines are the ones to alter depending on your requirements:
#define LED_PIN 5
#define NUM_LEDS 50
#define BRIGHTNESS 64
#define LED_TYPE WS2811
Change these to match the string of LEDs you are using.
In my video example below, I'm using Pin 5, 22 LEDs, 100 brightness and a string of WS2811 addressable LEDs.