You would enter that just as if you purchased it through a bank. Except you may not receive a Form 1098, Mortgage Interest from the owner. I would ask for the amount of interest paid in 2016 and the property taxes paid in 2016 (unless you paid them personally).
You would start to enter your mortgage interest and if he/she does not issue a 1098, then you need to check the box that says "The info does not match my 1098." on the entry screen.
This will bring up some blank lines to write an explanation. You will need to explain you have a private mortgage, did not get a 1098, and give the amount of interest and the name and address of the seller/mortgage holder.