As you have probably already read, it can be a fairly complex endeavor and you should only do Seller financing IF you cannot get financing through a bank or lender. You will pay a higher percentage and if there's an existing mortgage on the house, you risk losing everything you have paid the seller If that seller defaults on the note themselves, even if you are paying them on time.
If your credit is fair and you have a decent down payment, please give me a call and I will be happy to speak with you and refer you to a lender who can assess your situation at no cost to you. If you decide to do this on your own, I would be very, very careful.
Joe Jarusinsky, Realtor/Master Instructor, Keller Williams Realty, Austin's #1 Real Estate Company, Ranked #1 by Buyers and Sellers (JD Power & Assoc. 2012) Call 512-261-4415