How do I make my single-family house hack legit?

[Brian Burwell]

I've been housing-hacking my first home by renting out two of the spare bedrooms to friends for the past year. The amount I receive in rent covers most of my mortgage payments, allowing me to save most of my income which I plan to use for more properties in the (near) future.

I haven't set any systems in place to make our arrangement seem formal. Basically, I send them a Venmo request on the 1st of each month, they accept it, and I transfer the funds to my bank for automatic mortgage payments on the 5th. These payments have been consistent from when we first moved into the house, but there's nothing on paper to explain it (besides the subject/note on the transactions in my Venmo history)

I am certain that I'm shooting myself in the foot somehow. From an outside perspective (like from that of a future lender), I can see how this could look less-than-legit. 

So I guess my questions are:
   1. How am I creating problems for myself now and in the future?
   2. What actions can I take to not create those problems?
   3. How do I take what feels more like "my friends helping out with the mortgage" to something that's more legit?

Thanks so much in advance for your answers, advice, and suggestions!