How to write a letter for the bank that explains that I get paid cash?

Can anyone help or give me an example on how to write a proper letter singed by my boss, explaining to the bank that i get paid cash. Im trying to get a loan for a car but i work at a hair salon and they pay me cash the bank is asking me for check stubs so they can prove a form of income but i do not have any. they told me to write a letter but i have no idea how to start or what to put in it. someone help please!