Introduction and first question.

Hello. I'm a new user and an absolute tyro with routers. I may be asking lots of questions and I ask your forebearance and patience with what may seem to be very basic questions. I own a fairly new Freud router and an old Craftsman (before many turned to junk). I'm a retired Naval Aviator, 73 yrs. old. Married 50 years last Nov. Live on a small farm in SD. Hope to build my own casket with wood salvaged from the old barn on this place. My first question will be on dutchmen or patches. How does one match up the size of the patch with a template?