G: TL;DR: Add 1 tablespoon of acid (lemon juice or vinegar) to a measuring cup, fill to ½ cup with milk, wait 5 minutes.

Japanese (Grade-1-style inline):
すっぱい みず（juice）を まぜる — Add a sour liquid
ミルクを いれる — Put in the milk

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Direct answer (no fluff):

If you have lemon juice or vinegar

• Put 1 tablespoon of white vinegar or lemon juice in a measuring cup.
• Add milk up to the ½-cup line.
• Stir. Let sit 5 minutes.
  This gives you the standard buttermilk substitute.

If you have cream of tartar

• Use 1½ teaspoons cream of tartar.
• Add it to the ½ cup milk.
• Shake/stir like hell. Let sit 3–5 minutes.

If you have yogurt or sour cream

Also valid if your recipe tolerates slightly thicker dairy:

• Mix ¼ cup yogurt or sour cream with ¼ cup milk.
• Thin more if needed. This behaves almost exactly like buttermilk in baking.

If you’re out of everything acidic

Last-ditch:

• Add ½ teaspoon baking powder to your dry ingredients (to compensate for lost acid).
• Use plain milk.
  Not ideal, but keeps the bake from going flat.

If you tell me what you’re making, I can optimize the choice.