To permanently add a charm:
With the Xbox version, just select a piece of equipment with one or more empty charm slots in the inventory and hit A; a menu will come up allowing you to permanently charm the item with any of the charms you have collected in your inventory. In the equipment menu you can hit Y to add charms to equipped items (currently that is the only way to charm a bow).
On the PC, right click a piece of equipment with an empty charm slot.
