To make leaf powder: Leaves, stems and tops of plant should be dried in a cool dark place so the leaves don’t blanch. It takes a day or so. Then grind them into a powder using a clean spice grinder.
Store in a glass airtight jar in a cool dark place. It keeps well so this is an economical way to preserve and enjoy greens in winter.
(The powder will cling to the sides from static if storing in plastic.)