The process for adding your KTN to your reservation is dependent on the airline you fly. Every airline's website is different. In general, if you have a frequent flyer account with that airline, you can add it to your account details. That will carry over to any reservation you make with the airline. You can also add it when you make the reservation. It will ask you at some point during the process.
Do make sure you use the same exact name on tickets / FF accounts / passport. / Nexus . If these differ, you might not get Precheck.