What is the proper documentation for private funding for a flip?

[Kevin Stalbaum]

I will be purchasing my 4th flip and for this project it's my first time borrowing funding. Purchase price is $150k of which I'll be borrowing $100k from a friends family member. What is the best legal agreement in this case that covers both parties interests? Secondarily, looking for advise how to properly handle the form a tax perspective? I want to make sure that I have what I need to write off the interest expense. Thanks in advance!