This article is from the Care of Orphaned Kittens FAQ, posted to rec.pets.cats newsgroup. Maintained by Sharon Talbert with numerous contributions by others.
I recommend emptying the kitten both immediately before and about 15 minutes following each feeding. With any luck, you have already emptied the bladder. Evacuation of the bowel will probably not happen at the first attempt and may take a day or two. When it does happen, don't be horrified at the toothpaste consistency and mustard color -- this is normal for a newborn. (A grayish stool is cause for concern, however; call the vet at once.) Once bowel movements have begun, you should see a movement for every feeding.
Kittens dehydrate quickly, so feed carefully to prevent diarrhea. Do not over-feed and do not make sudden or radical changes to the kitten's diet. If diarrhea (or constipation) develops, consult your veterinarian for adjustment of the formula or feeding portion. If the stool is liquid or bloody or contains mucous, consult your vet or make an appointment.