It kind of depends on the cow. If she is an excitable sort and never been tagged before, put a halter on her and tie her head to the side of the chute, short (otherwise she could throw her head and rip her ear or nail you). If this is your first shot at tagging, I would recommend a halter anyway. Then load your ear tag gun, grab the end of her ear, position the tag about 2 inches in (maybe, I can't judge distance so I could be wrong on that last part), in the center of the middle web and let her fly. If she has already had a tag, you can simply re-tag using the existing hole.