I've several chihuahuas. First of all, keep the puppy very warm. Not too hot. Wrap a hot water bottle or heating pad in a blanket then lay the puppy on it. Keep it in a box and try to have the mother feed it. If she's not willing to feed the puppy. Take some baby formula or 2% milk and warm it up just like you would for a baby. An eyedropper works great for these small dogs. Feed a few droppers every 2 hours or so. (Just keep thinking about a newborn baby and how to care for it. It's basically the same feeding pattern.) After feeding, if the mother's not doing it, take a piece of toilet paper and dap the butt and pee area because they can't "go" by themselves when they're this young. Good luck!