1)What to Feed: Critical Care (or a pellet mash if CC is not available) works best for babies. Try adding a bit of cooked pumpkin (or butternut squash baby food) to the primary food, either Critical Care or crushed pellets. Feed small amounts frequently during the day (the more frequent the better, perhaps every 1-2 hours, at least every 3 hours). At first you may only be able to feed 1 or 2 cc of pellet mash every hour or so. Carefully syringing water (or unsweetened pedialyte) can help keep the pup hydrated.
2)No Milk Products: Milk products and milk replacer products are not appropriate for guinea pigs (milk). None are formulated to replace a mother cavy's milk. Cavies are quite mature when born (long gestation), so that while a guinea pig will benefit from its mother's milk, milk is not necessary to thrive and survive.
3)Technique: Some pups will eat from a spoon. Others need more aggressive hand feeding. Feed carefully to avoid aspiration (getting fluids into the lungs vs. the stomach -- may cause pneumonia).
4)Encourage Eating Standard Foods: Make sure pellets, hay, water, leafy green vegetables, and grass are always available so the cavy can begin eating on its own as soon as possible.
5)Elimination Tips: You will need to help orphans to pass waste during the first week or so of life. Normally a mother will do this by cleaning the pup's genitals. Wiping the genitals after feeding will help to stimulate elimination. Try a warm, wet washcloth, stroking several times.
6)Foster Mothers: A foster mother will often accept pups. Pairing a mother with few pups with one that has many gives the pups from the larger litter a better chance at survival.
7)Time Alone: Some runts can benefit from extra time with their mother. Remove the competing pups periodically for 15 minutes or so to give the runt more opportunity to feed.