how do I make sour milk for baking

What others have said. A tablespoon of something acidic (lemon juice, vinegar) added to milk will, in effect, make it buttermilk in taste.

I've always put 2 tablespoons of fresh lemon juice (or apple cider vinegar) into 1 cup milk.

My way is different than above. For 1 cup of Sour Milk I usually substitute I cup plain yogurt or 1 cup evaporated milk + 1 tbsp. vinegar. If I need 1 cup of buttermilk I substitute 1 tbsp. cider vinegar or lemon juice + enough milk to = 1 cup.

Also, let the mixture stand for about 5 minute, then use in recipe.

1 Tb acid (vinegar, lemon juice, etc) mixed into 1 cup milk