You have to figure in the cost of the halter (if you provide it), the cost of your materials (will vary based on design) and a little bit for your time. You just advertise wherever - message boards, craigslist, ebay - and people will see. You could also do a local service of charging to gypsy-up someone's existing halter, in which case you just charge for materials and time.
Did you hand sew a ribbon on and then make tassles? I think it's super cute!
* I'm often reading and posting from mobile and Siri loves to make a mockery of the English language.