BUILDING MY OWN DRONE FOR MAPPING WITH PPK OR RTK

Hi, i'm working in the aerophogrammetry business for construction with my phantom 4 pro, but know i need an fixed wing to start working with agriculture.

I want to build my own drone, and i saw this range video(rvjet) and the believer 1960 frames, know i need to know how to build the drone.

Can someone help me to begin the project?