Owner finance need to figure out contract.

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First time on BiggerPockets & first post. 
i’m working on a deal the homeowner wants 100,000 for the home asking for 50-60k down homeowner will finance the rest any recommendations on how to write up a contract for this? 
  

Do we involve a lawyer a banker don’t know where to start. 

Thanks for the help and any information someone can give me ! 

Matt