Loan Agreement for use with Private Lenders?

Hi guys!! It's been a long time. I have been in talks with a friend of a friend who happens to have some money collecting dust that he'd like to put to use. I'm in the middle of a flip using an HML and I'm about to start another, I would LOVE to go the private route and save some points and interest. I looked in the forms and docs and I dont see anything that would help legalize a loan from a private lender... does anyone have any info on this? I'm basically just going to do a 6 month term for 10% interest, with the interest rolled in to the loan. I want to make sure it is 100% legal and that I don't have problems come tax time, with them wondering where the money came from. Is it best to do it like a standard loan, or should it be secured by the property like a mortgage? Any advice on this matter from someone with experience with private lenders would be greatly appreciated! Thanks in advance!