August time, and wait until the leaves drop and turn yellow.
On a dry day lift them with a fork, and leave them to dry on the earth in the sun for a good few hours.
You can dry them off by putting them in a sunny place, undercover, but not too hot - I put mine on a slatted frame, bulbs upwards, and the leaves hanging down. The warmth in the air will gradually dry them.
So when the leaves are dry, plait them up in a bunch and hang them in your garage or shed where it's dry.
Just used my last ones from last years harvest the other day.