I looked at the tutorial, and it should still work, though I've never tested it with GLES2. Is the issue that the texture is not showing when the project starts? If so, it might be because of assigning the ViewportTexture in the Godot editor. I have found that doing it manually through code is generally easier and less error-prone.
Though I agree with @Megalomaniak, knowing what is not not working would be helpful!