First, create a new layer and place it at the top-most level. You can also rename it to something like "Fade". In its properties, locate the "Transparent" one, and set it to "No". Next, set the "Background color" property to black (or any other color you wish the fade to be), and finally, set the "Opacity" property to 0.
The actual fading can be done in a number of way, but the easiest one is to lerp from 0 to 100 using an incrementing lerp factor. Like this :
The "Start of layout" can be replaced by any trigger you want.