Fill a full sized pint glass (or bigger) half way with cider, then slowly pour the stout over top to get a layered effect (a la Black and Tan[1]). Using a spoon to pour the stout over can help diffuse its momentum and create more defined layers.

This receipe works best if the stout is Guinness and the Cider is Strongbow.

[1]: Black and tan, http://extratasty.com/recipe/34/black_and_tan