Asking the seller to carry the down payment

Hi all, I am talking with a seller about selling off-market. She is asking market rate for her property so I wouldn’t be getting a great deal in that sense. I was curious how having the seller carry the downpayment of 25% would look like? If I obtained a conventional mortgage of 75%, how would the other 25% be paid? Separately to the seller with an agreed upon interest rate and terms? Is there any special paperwork for that?