Horsetail herb harvesting is done in the summer. Pick stems, discarding any discolored ones, and let them dry in a cool, dark place. Once dry, the stem can be ground into a powder and stored in an airtight container for up to one year, or use for ornamental purposes. Young shoots can also be eaten like asparagus.
Another site has instructions for making horsetail herb tea…and plenty of other good information:
http://www.superfoods-for-superhealth.com/horsetail-plant.html