A black and tan is a light lager with a stout poured on top. As Dr. K. is Irish, that means Harp Lager and Guinness Stout.
Fill the glass halfway with the lager, tilting the glass as you pour to minimize the foam. You need a special tool next, not to worry, it doesn’t cost much, unless Dr. K. uses one of your expensive spoons.
Bend the handle 90 degrees upward to allow the spoon to fit into the glass. (that’s the expensive part, if you let your husband go scrounging around in the cutlery drawer to suit his whim. . .)
The tip of the spoon should be at the top of the lager. Pour the stout onto the spoon to prevent mixing, slowly raising the spoon as the glass fills with that dark delight.
Don’t let the spoon get into the foam, or you will be left with a brown beer instead of a black and tan.
Repeat above twice, unless you are Irish…