Advice on keeping track of payments w/ owner finance property

[Estela Nava-Hinojos]

We are looking into purchasing an investment property that the seller is willing to finance but my question is what is the best way to keep track of payments made and amounts owed, interest, etc. We will have a 5% interest rate after 15% down (not sure that matters). I'm just curious how payments would be made and where interest would go since there is no bank financing? Am I overthinking this or are there mortgage/title company services available? Thanks in advance! New to the group and love the community!