What if you're Self Employed?

Income is income...   You just have to prove that you have it.    In general, documents that you'll need to present might include:

1. Tax Transcripts
2. Current bank statements – checking & savings
3. Landlord reference letter stating the term of your lease & payment history
4. Cancelled rent checks for the last three (3) months of your lease
5. Legible copy of valid state photo ID, driver’s license and/or passport

Some possible ways to prove cash income:

1. Create paystubs* 
2. Keep an updated spreadsheet
3. Use bookkeeping software
4. Deposit payment and print bank records
5. Put it in writing with client signature
6. Create own receipts*
7. Utilize tax documents
8. Use an app
9. Proof of income letter
10. Employment contracts

*I don't mean create fake receipts and pay stubs.  I'm referring to real ones.  
Please note, I am not an accountant, bookkeeper, lawyer or financial advisor.  Please seek advice from the appropriate professional.    Tell them you need to document your income and ask them to help you create a system.

If all else fails, you can also use the services of a personal or third party guarantor.   In that case, you will usually need to prove that you make about 27 times the monthly rent.  (A personal guarantor would have to make 80x the monthly rent on an annual basis.)