How do you take care of a newborn baby possum?

I would not give it or any baby, kitten or puppy, cows milk. In the case of a possum, crush up some high protein cat food, dilute with small amount of water and give it in an eye dropper. Dried mill worms would work too, available at pet stores in the lizard/snake food section. No milk. Yogart, gatorade, strawberries, blueberries.

feed it warm milk and raw meat

Get a baby animal milk replacer, and a syringe, and let the liquid go slowly into the side of its mouth. Give it some diluted Pedialite as well, and put it in some sort of small mammal sack, with a Snuggle Safe to keep its body temperature up. ( You can get these at PetCo).