Motherless kittens have two vital requirements: a suitable environment and appropriate nutrition.
Kittens need a warm environment, which in the first weeks should be controlled by a heating lamp or a hot water bottle wrapped in blankets. For the first three weeks of life, small kittens must be stimulated to urinate and defecate. For stimulation, the mother licks the belly and the anus of the kitten with her tongue. You can simulate the mother's behavior by gently rubbing a piece of warm, damp cotton batting (which feels like the mother cat's tongue) on the anus, genital area and abdominal wall. Who said caring for cats didn't have its privileges?
Feeding orphaned kittens is a challenge. Kittens under one week of age need to be fed six times a day or every four hours, day and night. Cat milk is higher in fat and proteins than cow or goat's milk, so don't feed "regular" milk to orphaned kittens. You should get commercially prepared milk substitutes specifically designed for kittens, from a breeder, or from your veterinarian. Feed the kittens slowly with a small bottle. Be sure to prepare the milk substitute fresh for every meal, and warm it to about 100.4 degrees F (body temperature).
At around three weeks of age, kittens start to explore their environment more. They should now be able to lap their milk substitute from a dish and even nibble solid food. Give young kittens as much food as they like -- they rarely overeat at this stage of their life. Use highly palatable calorie and nutrient-dense kitten foods.
At about eight weeks, the kittens will be fully weaned.