How to Air Dry Wild Garlic:

Thread a long, sturdy piece of string through a large sewing needle. Stack a few leaves at a time and carefully pierce them near the stem end. Keep going until all the leaves are strung. Hang the string somewhere dry and airy in your home. Spread the leaves out along the string so each one gets enough air. At my place, they took about three weeks to fully dry.

How to Dry Wild Garlic in the Oven:

Spread the washed leaves on a baking sheet lined with parchment paper. 
Put it in the oven at 40°C / 100°F.
If your oven door fogs up, wedge a wooden spoon in the door to let moisture escape.
Every now and then, turn the leaves over and check how dry they feel. They’re done when they rustle and crumble easily.
Mine took about 4 hours to fully dry.

Crushing and storing

Once your leaves are completely dry, crush them with your hands. Then finish it off with a mortar and pestle. Store your wild garlic seasoning in an airtight jar, away from light.