I use a very fiddly process which requires taking the pieces of the ship livery and placing them so that they resemble something like the ship that I'm using. You can see it in the image I've attached below. After that I overlay an image and cut it so I can then rearrange the ship pieces to where they were on the original livery map. I've used this method three times on my AG-Systems Japan livery and Piranha and Icaras Union Jack liveries.