Formality of an offer

[John Snow]

After overworking every number possible in a possible flip property, I'm ready to make an offer on a property. I'll be meeting the homeowner Monday to discuss that offer. I came across this property prior to it being listed on the market, they're planning on listing it at the end of the month, and I have no realtor involved on my end. The homeowner has a realtor that will be listing it, if it gets to that point, that she would like to have be a part of the conversation. My question is, how formal should this offer be? Documentation? I have been a contractor for the better part of the last 30 years and most of my deals were the product of a "discussion" and then we formalized it with contracts at a follow-up date. Can, should, I approach this the same way or should I be prepared other-wise? Thanks in advance!