The mix I used was 1 part dry puppy milk with colostrum (or kitten, different people have different preferences) to three parts warm water and one part heavy cream. My vet recommended adding 1 part goat milk also, but the babies were already used to the mix I was giving and I didn't want to change it.
For dehydrated babies, use unflavored Pedialyte and just add a tiny bit of milk to give it some flavor so they take it better. It's important to rehydrate them FIRST, or they won't be able to digest food well. Sub-q fluids would be best if you can do it.
I fed from the smallest oral syringes I could get. They're Britannia Petites so they had super tiny mouths. I tried nipples from the kitten nursing bottles, but my babies had already forgotten how to suckle. They forget pretty quickly, I guess.
Here's some helpful links that I found while raising Dora. Eventually I'll have a Library thread on raising orphan babies:
http://www.2ndchance.info/bunnies.htm
http://www.bio.miami.edu/hare/orphan.html
http://www.islandgems.net/handfeeding.html
http://www.kindplanet.org/rabbitbabies1.html
http://jubileeacres.net/bottlefeeding.html
You can also get some specially-made milk that wildlife rehabbers use, as well as feeding equipment:
http://www.foxvalleynutrition.com
http://www.squirrelsandmore.com/index.cfm
Also, keep Benebac on hand. I'd give some every day even for healthy babies. I... didn't have healthy babies.:rollseyes Do be aware that hand-fed rabbits are more likely to have GI issues, sometimes severe. It helps if they can at least get colostrum (first milk) from their mom.
In cases of severe diarrhea/enteritis, cecals from a healthy rabbit are a lifesaver. Fortunately I was fostering a different litter and their mom at the same time and the babies left little cecal presents for me everywhere. I'd take one, mash it up with some Pedialyte and a tiny bit of milk, and make the bun swallow at least some even if they think it tastes gross. Probably saved Dora's life.
One more tip: Snuggle Safe! Best thing ever, especially since I was dealing with two dehydrated, scrawny babies with no nest. If there's a good nest a number of babies they'll probably be ok although I'd add some supplemental heat if they seem sickly at all. Make sure they can move away from it in case they get too warm.