An EEG's not bad. Trust me. I had them done for the first half of my life. I never heard the no caffiene rule though or the sleep one. Basically, they take wires and glue them to your head and clip two to your ears (one on each lobe). The'll either put you in a recliner or on a bed and most of the ones I had have you go to sleep and someone in the other room measures your brainwaves. It's a pretty noninvasive procedure, the only thing I never liked is the glue is hell to get out of long hair.
BTW, if you wear earrings you have to take them out. Good luck tomorrow. If you don't mind me asking, why are you getting it done?