Did the mother pass away or did something else happen?
You should check out this website. I have never bottle fed a puppy solely, but my puppy was on supplemental formula when she was still with the litter and the mother was an underweight stray and a mother of 9. She took the bottle very well from about 3 Weeks on.
Start off with a syringe with a puppy that small, they may not take a bottle too well at first (especially a baby bottle, that is probably too big).
Get veterinary care asap, if you haven't already. A puppy that tiny is susceptible to all sorts of infections.
You need top be wiping the tummy and genitals with a warm cloth after feeding to stimulate digestion and to stimulate them to go to the bathroom. Leerburg | Bottle Feeding Puppies
ETA, I don't know offhand how often you need to feed them (maybe another member can help- paging Tess!), but this with be an around the clock job for the next few Weeks, don't expect to be getting much sleep!