Give the item a dummy ability with no stat bonuses (unless you want the item to give a bonus).
Item Armor Bonus is a simple enough ability to use. Set its
Data - Defense Bonus to 0 and change its
Art - Target to whatever you want the attachment to be.
Art - Attachment Point 1 can be wherever you want the effect shown. Some examples:
head
chest
hand right
hand left
weapon
overhead
origin
Here's a tutorial if there's any confusion:
Attachments and Weapons