To make milk for a kitten, you need to buy powdered milk replacement designed for kittens and then mix it together with water in the amounts listed on the label. Heat it up to slightly above room temperature before giving it to the kitten.
Other forms of milk are not nutritionally complete and cannot keep kittens healthy. If you have an orphaned kitten that needs to eat but do not have any milk replacement available, mix condensed milk with an egg yolk and yogurt to make an emergency formula. Do not use this as a long-term solution. You can also mix regular cow or goat's milk with egg yolks as an emergency formula, but the lactose can make kittens sick because they cannot digest it. Try mixing unflavored Pedialyte with the formula instead of water if the kitten has gone a long time between feedings and is dehydrated.
Although sealed milk replacement formula can usually be stored at room temperature, opened containers need to be kept in the refrigerator, or they can spoil. Do not leave mixed formula out for too long before using it.
If you have to microwave the milk before giving it to the kitten, make sure it is not too hot. Check for hot spots as well, since microwaving does not always heat bottles evenly.