How to do Singleton scripts in Unity

Singleton.cs

public class Singleton : Monobehaviour
{
  private static Singleton _instance = null;
  public static Singleton instance
  {
    get 
    {
      if (_instance == null)
      {
        // First try to find an instance already in the scene
        _instance = GameObject.FindObjectOfType<Singleton>();

        // Only instantiate if we're not cleaning up 
        // othewise objects will be leaked into the editor's version of the scene!
        if(_instance == null && !_shuttingDown)
          _instance = new GameObject("Singleton", typeof(Singleton)).GetComponent<Singleton>();
          
        // Let the user know we're returing null
        if(_instance == null)
          Debug.LogError("What is dead may never die!");
      }
      return _instance;
    }
  }

  private static bool _shuttingDown = false;

  void OnApplicationQuit()
  {
    // This is important: we need to prevent instantiation during scene cleanup!
    _shuttingDown = true;
  }
  
  void Awake()
  {
    if(_instance != null)
    {
      Debug.LogError("There can only be one!");
      DestroyImmediate(this);
    }
    else
    {
      // if a job's worth doing it's worth doing twice... just to make sure
      _instance = this;
    }
  }
}