At its most basic, nasal snuff is simply fully cured and aged tobacco leaf, pulverized to fine or coarse dust. A screen can be used to remove any larger bits or pieces. Some have used a burr mill coffee grinder to pulverize the leaf, after stemming it. The variety of leaves used is your choice.
It can be used unflavored (or unscented), or essential oil can be added to modify its character. This is usually dissolved in water or plain alcohol (e.g. vodka), then mixed into the powder. For nasal use, it needs to first be fully dried.