Those blanks get filled by a real estate attorney or, in some states, by the closing agent working from attorney-approved forms, and how the documents have to be worded and recorded differs state by state, so this is a question for someone licensed where the property sits.
Rough costs people budget. Attorney drafting of a note plus security instrument commonly runs a few hundred dollars to around $2,000 depending on the state and how custom the terms are. Recording the security instrument is usually a modest county fee, sometimes tens of dollars, sometimes more where transfer taxes apply. A third party loan servicer typically charges a setup fee in the range of $75 to $200 and then something like $20 to $40 a month to collect payments, track the balance, and issue year end interest statements. If the buyer is going to occupy the home, federal rules on residential mortgage loan originators can require a licensed RMLO to prepare the file, and that service often runs a few hundred dollars up to about $1,000 per loan. Whether it applies to you depends on how many loans you make and on state licensing rules, so confirm it in writing with a licensed professional before you sign anything.
The cost most first time carry sellers forget is monitoring. You need to know if the buyer stops paying property taxes or lets the hazard insurance lapse, because a tax lien can climb ahead of your lien and an uninsured fire wipes out your collateral. Servicers sell tax and insurance tracking as an add on, usually a few dollars a month, and it's cheaper than finding out late.