If this is your first time doing this, then I would recommend using something small to purify, such as a ring, or a charm. (I would use a charm.) Light your incense and say:
"Goddesses of the elements, hear me. With this incense I hear by purify this object. May it be blessed with spiritual power. So mote it be."
As you say this, wave your object slowly through the smoke. Make sure that the smoke is covering every inch of your item. Also, as you wave you item through the smoke, silently chant the elements until the incense burns out by its self. This may take 30-45 minutes depending on how long your incense is, so be patient. I hope this works for you, and blessed be!