How do i explain the source of earned income or wages I used to compute my earned income credit? What documents do I send to support my entry?

Received form 3531 request for signature or missing information to complete return. Box 1 checked and box 7 checked.

I am a household employee for several households whom supply all materials used to complete tasks under their direction, earning no more than $2000 each household. I work off referrals from households I've worked for.  I was paid in cash during the course of employment with each household. The IRS wants me to explain the source of earned income or wages I used to compute my earned income credit and to attach documents to support my entry. What type(s) of documentation would I need to send them. Also, where do I explain this to them?