Microwave the zest in a microwave-safe container (preferably flat so that the moisture can evaporate quickly) at 10-second intervals until dry. Be sure to periodically stir the zest so that it dries evenly. When dry, let cool to room temperature.
Grind in a spice grinder.
Store at room temperature in an airtight container. If it is completely dry and well covered, it can last up to 1 month.