You should have a contract and the terms prepared by an attorney, agent and/or mortgage professional. Among the terms will be price, down payment, rate, term, late fees. Will the title be transferred to your name with them as lien holder or will they retain title until you replace the loan?
Commonly seller financing is for a limited term like 2 or 5 years with a balloon note due for the unpaid balance. What if you don't qualify for a loan at that time? Will the rate be fixed or subject to change? Are you allowed to modify the home, as in remodel or improve it without their prior consent?
These are just a few things which come to mind for starters. You don't want to do this on your own, get professionals involved for all concerned. A well thought out, explained and prepared set of contracts are beneficial for all involved. Best of luck.