Rocker panels, and cab Corners On 84

[BirdFlewLS1]

I am new to this site and new to the body aspect of wrenching. so i was wondering How do i go about replacing my rocker panels and cab corners on my 84 chevy truck. Ive been told to take the doors and fenders off and cut away the rust on the cab corners and then fit it and trace it and cut about an inch below the line...which im not understanding because then there would be a edge? i dont know im probably not thinking it threw...and pictures or help would be awesome