First — eat where the queue is local. If a kachori shop has more visitors than residents, walk past it.
Second — eat in season. Malaiyyo is winter only, late November to early February. The cloud-light dessert that Varanasi makes from cold-night condensation will not exist again until next year. Lassi is summer, served thick in clay cups that you keep.
Third — finish with paan. The paan stalls of Banaras are an institution. Order a 'meetha paan' if you want sweet, or 'saada' if you want plain. Either way, fold and chew for ninety seconds and swallow.
Fourth — don't trust 'Banarasi' on a menu outside Varanasi. The name has been borrowed everywhere, the recipe rarely.
Fifth — eat slowly. The whole point of the city's food culture is that nothing is in a hurry. The shops have been here for ninety years; the next bite will not run away.