I've been away for a while. Where are people getting liveries these days? I remember we used to have a thread where you could post your idea and people with the proper skills would generously build one for you. I suspect the answer is AI but I am extremely opposed to that option and much prefer a more home-made aesthetic.
The simplest manual way is still to download one of the livery templates and edit it using a paint program that supports .psd files / layers (Photoshop, Gimp etc)
The biggest public resource for templates is currently here:
https://airline-club.fandom.com/wiki/Airline_livery_bases
Though a few links for more niche type have been down for ages.
After that you can even ask AI to put your designed livery onto an actual photo!