YouTube Video Tutorial: https://www.youtube.com/watch?v=et6BAdlxECw&t=7s

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class BillboardFX : MonoBehaviour
{
	public Transform camTransform;

	Quaternion originalRotation;

    void Start()
    {
        originalRotation = transform.rotation;
    }

    void Update()
    {
     	transform.rotation = camTransform.rotation * originalRotation;   
    }
}

Script Link: https://github.com/ThisIsFix/Unity-Billboard