LED Objects can be used with the AddressableLED class in the edu.wpi.first.wpilibj package.
AddressableLED constructor : public AddressableLED(int port)

In order to actually modify the LEDs, you will need to use the AddressableLEDBuffer object found in the same package.
AddressableLEDBuffer constructor : public AddressableLEDBuffer(int length)
Length refers to the amount of LEDs, or "pixels" on the LED strip.

Then, you need to set the length (amount of LEDs) of your AddressableLED object using the respective setLength method. Note that you should never ever call getLength

And finally, you need to set the data of the LED using AddressableLED#setData(buffer), and initiate it with AddressableLED#start.

// The following code is a snippet from the LED subsystem I wrote.
private AddressableLED led;
private AddressableLEDBuffer buffer;

private static final int port = 0; // The PWM port of the LED strip
private int length; // How many leds, or "pixels" are on the strip

public LEDSubsystem(int pixels) {

	this.length = pixels;

	buffer = new AddressableLEDBuffer(pixels);

	led = new AddressableLED(port);
	led.setLength(pixels);
	led.setData(buffer);
	led.start();

	led.setData(buffer);

}

Actually using the LEDs is very easy. The AddressableLEDBuffer class has various methods to make your life much easier.

NOTE: You need to update the AddressableLED's data after making changes to the buffer!

^{_{LEDs are very cool. 👍👍👍}}