Kitten getting "the snip"

Hi OP, don't worry about this procedure, you're doing the responsible thing in having her neutered.
If you're particularly worried about the anaesthesia, maybe ask the vet to carry out a 'Pre-Ga' blood panel before she has the anaesthetic administered, this test checks for kidney / liver function, which are important to for the metabolism and elimination of anaesthetic drugs in the body. Its very unusual to find any abnormalities in young animals, but just for peace of mind it's not a bad idea. It can identify problems that aren't yet apparent, and the drugs / procedure can be altered accordingly.
I probably wouldn't be inclined to give her an 'extra big' feed on Tuesday to keep her going, just feed her her normal amount that you normally would, but nothing after about 9pm, obviously leave water available for her at all time. The reason that animals are fasted before a procedure is, like in humans, it decreases the risk of vomiting under general anaesthesia and it also prevents a full stomach from pressing down onto the diaphragm and hindering respiration. The vets aren't doing it to be mean! She can have a small meal when she comes home if she's up to it just to tide her over for the night.
Depending on the surgeons preference, she'll most likely be spayed lying on her side, and will have an incision on her flank, she'll also have some hair clipped up from the area too, but it'll grow back very quickly.
The most important thing that you can do for her when she comes home is keep her indoors (for the duration that the sutures are in place) and nice and warm, particularly the night she comes home. Her depth perception is likely to be affected for a few hours post-anaesthesia, so no jumping onto furniture, 'mad-five-minutes' etc.
She'll be a bit groggy and quiet for a day or so, but most are back to themselves after that. Keep a close eye on the wound, monitor it for swelling, discharge (bloody or otherwise), gaping, inflammation etc. If she's bothering the wound, you may have to resort to using one of those 'buster' collars to prevent her from biting at the sutures.
Can't think of anything else TBH, its a relatively straightforward and very commonly done procedure, she'll be fine!!
Sorry for the long post

Well, the pre-ga testing is basically a general panel for kidney / liver function including blood glucose. Its gives a good overview of general organ health, but if more specific diagnoses are needed, more specific testing can be done.
It is usually a good indicator of problems in the organs prior to the administration of certain drugs. We do these tests very frequently in my clinic before surgical procedures, primarily in older animals with whom we would expect or anticipate more systemic health problems, but more commonly in young animals undergoing routine procedures too, just in case
Elevated readings in these tests do not automatically mean that a patient has a certain life threatening disease, they can be altered due to many things, like stress, the animals life-stage, current medication whether or not the animal has eaten etc.. but, in conjunction with clinical signs and medical history, they are usually a good indicator of general health if the readings are all normal.
Some of the tests that make up the pre-ga panel are indeed some that you would use to diagnose kidney disease or renal failure, but you would be more likely to make a diagnosis based on a series of tests over a few days, as one elevate Urea sample (AKA blood urea nitrogen) may be caused by dehydration and can be corrected with intravenous fluids. However, if over a few days on IVFT, the values are still rising then a diagnosis of kidney failure is more likely. You would always consider other symptoms and clinical signs too before making a diagnosis like that.