Create a Scene Loading Progress Bar in Unity 3D

 Quite simple actually... use the Slider from the UI and be aware that an AsyncOperation of progress value 0.9f means that it's done, at least when it comes to loading a scene.

Here's the code:

public Slider LoadingProgressSlider;

private void Start()
{
    LoadingProgressSlider.gameObject.SetActive(false);
}

public void GoToScene(string scene)
{
    StartCoroutine(LoadingSceneRealProgress(scene));
}

AsyncOperation _sceneAO;
IEnumerator LoadingSceneRealProgress(string sceneName)
{
    LoadingProgressSlider.gameObject.SetActive(true);
    yield return new WaitForSeconds(0.5f);
    _sceneAO = SceneManager.LoadSceneAsync(sceneName);
    _sceneAO.allowSceneActivation = false;

    while (LoadingProgressSlider.value < 1f)
    {
        LoadingProgressSlider.value = _sceneAO.progress / 0.9f;
        yield return null;
    }
    _sceneAO.allowSceneActivation = true;
}