Hi there, long thread so I won't be touching on everything, but you should definitely take the bird to an avian vet to learn what to do if you have a neonate but don't know how to handfeed it. If you live some place where an avian vet is not available, try to find a local bird store. They will most likely be very happy to help you. In fact, even the "kids" you see working at many bird stores can be a big help, I remember my first job as a young guy at a bird store in Las Vegas I learned to handfeed by having a batch of 30 babies from multiple clutches all at once, so you'd be surprised how much experience some of these young guys and gals have at these shops.
As for feeding amount, typically 10% of the birds body weight (weighed in MIXED formula) is ideal, I say ideal because some birds will be finicky eaters until they get used to you. What I mean is, it's not uncommon for a bird to be eating fine with one feeder, but then be a struggle for a new person, especially someone that is just learning. There are spoons, gavages, pipettes, infusion cannulas, tubes, etc - LOTS of different feeding devices, let alone the fact that some people feed at different consistency, different temp (102-110 being most common) etc. The baby can form preferences so that's usually what the issue is with most people when they "take over" a baby. Here is where some people will disagree, but I actually recommend feeding fairly fast when possible in these cases. The thing is, most breeders, jobbers, pet shop owners etc are professionals that can feed MANY babies per year (like I said, my first handfeeding job as a kid at a pet shop was 30 cockatiels at different ages all at once). The problem is that while this isn't necessarily a bad thing, the baby can become conditioned to a quick style of feeding, and when someone tries to feed slowly, the bird takes a breath while there is still food in the mouth - This is bad. So, most people find that when they are taking over a baby sold wholesale or just bought from someone that has already been working with it, that the best thing to do is feed warm and fast.
Also, I happened to see someone mention the issue of birds begging even after feeding or during weaning. When you are handfeeding, your job is to watch the crop and the clock. They should be coinciding with each other. Most parrot species after about 2 weeks can be safely fed 4 times per day. My typical schedule was 8am, 12 pm, 4 pm and 8 pm. The bird should be empty or near empty at these feeding times. Here you also see that at this age everyone sleeps through the night (phew!). However, some species, especially birds like cockatoos and ESPECIALLY cockatiels will literally eat until you kill them if you listen to them and feed when they beg. Trust me on this. Over feeding is bad. Feeding too much starts to slow the crop down or stop digestion (it's called Crop Stasis) and this can be fatal as the food soon spoils INSIDE of the bird. And even if they just slow down and start being late or skipping feedings, this can also cause issues such as stunting, etc.
Finally, macaw formula was mentioned. The macaw formula is best used with macaws only. Old world species like cockatoos should never be fed this food. You can feed a macaw the same formula you'd feed a cockatoo, but you can't feed a cockatoo the same formula you'd feed a macaw. The fat content is much too much.
So, this was my first post in this group, how'd I do?
-Chris