Looking for 1st Seller Financing deal guidance

[Bryan Stely]

I recently viewed a duplex that I want to purchase and found out that the seller is open to seller financing. Not having done a seller financing deal before I don’t want to assume I know what I’m doing and more importantly don’t want to rush into it without trying to get some feedback from people who have experience on the topic. 

What is some good advice for a first timer? What are some documents that need to a part of this type of deal? If the seller still has a mortgage, should I asked to be an authorized user to pay the lender directly? 

Any advice is appreciated, thanks.