Oh my, you've really got "new cat ownership by fire" going on, don't you. Well, it's not ideal, but it's sure one way to learn how to take care of a cat, and you'll be amazed how much you learn about feline psychology while nursing an ailing feline.
Here are a few links that will provide you with lots of help in assist feeding your girl:
http://www.yahoogroups.com/group/Feline ... ed-Feeding
As others have mentioned, it can be very helpful to securely wrap the cat in a towel. I find it helpful, also, to use a large diaper pin to pin the towel closed at the back of the cat's neck. That helps free up both of my hands for the feeding. Note, however, that some cats fight more with that sort of restraint and may actually be easier to assist feed without towel wrapping. You just have to see what works best with your girl.
It's important to syringe food in sloooowly from the SIDE of the mouth ACROSS the tongue, NOT the FRONT of the mouth toward the BACK of the tongue. By syringing in small amounts from the side across the tongue, you give the cat time to swallow properly and not aspirate any of the food into her lungs (which can cause pneumonia).
Avoid making the food mixture too watery (too easy to aspirate). You'll have to experiment a bit to find the right consistency so that she can't spit it out easily but so that it's thick enough that it won't run down her throat without swallowing. You can make just about any type of pate-style canned cat food syringeable by pureeing it in a kitchen blender with a little hot water.
Try to get at least 20cc's into her per feeding, every hour or two. 40 or 60cc's is even better (much easier - and messier - said than done, I know). If she doesn't receive adequate nutrition, she is at risk of developing a serious liver disease called hepatic lipidosis. For future reference, a cat should never be allowed to go more than 48 hrs without eating before you take her to the vet. It doesn't take long for hepatic lipidosis to occur in susceptible cats.
As another poster mentioned, baby food meats can entice many cats to eat, but you must read the labels and make sure they don't contain ANY onion or garlic in any form (both of which are toxic to cats).
I would not recommend syringing plain water, as it is far too easy to aspirate into the lungs. If she does not drink on her own, go back to your vet and ask to be shown how to administer subQ fluids at home to keep her hydrated. Depending on how much food you manage to get into her, you may not be able to keep her adequately hydrated just with assist feeding.
If your girl is acting sicker tomorrow, I recommend you take her to an ER vet. I'm worried that she may have picked up a nasty infection from the spaying. Heaven forbid something may have even been left inside her during surgery. Hopefully, though, the food and hydration she received today will have her feeling much stronger by tomorrow.
Please keep us updated on the condition.