PJiang, BaiTu pls help - Self-Employment-EB3

I am trying to use AC-21 to do self-employment, what do I need to do? I have clients and projects, but they are cash based, how to do it? What type of company do I need to setup for myself? How to provide evidence if USCIS asks?

In the past, I just added them to my 1040, but now because I plan to use AC-21, I need to provide evidence in case USCIS asks for it, plz give direction on that.