Music Teacher (Private Tuition) - Job Offer Question

Need some advice to figure this out.

I am a Musician/Composer. Currently in New Zealand on a Silver Fern Job Search Visa.

Music Teacher (Private Tuition) is listed in the Skilled Job category in Appendix 6.

Does anyone know what sort of 'job offer' letter/contract will INZ accept? Considering Private Tuition doesn't have an employer and is self employed in nature.

I am unaware as to what sort of documents can credibly support my Visa application.

Thanks in advance.