First, you will need the right tools. Photoshop or Gimp is an essential tool for repaints.
Second, it is important to familiarize yourself with the file and folder structure of repaints. For this, please check out the "liveries" folder, and take a look at the existing liveries. Note their structure. The first sub-folder that contains the livery will bear the name of that livery. This folder name is what's used in X-Plane in the selection dialog. For instance, if a folder inside the "liveries" folder is called "Metallic Blue", then this is one option you will see in the livery selector in X-plane.
Inside the folder that has the name of the livery, there's a sub-folder called "objects," and THAT's where the actual image files for the repaint are contained. Each file that is contained in this folder acts as a substitute for a graphic file of the same name that is normally found inside your aircraft's main "objects" folder.
For instance, if in the "liveries>MetallicBlue>objects" folder you find a graphic file with the name "fuselage.png", this file will be loaded, instead of the default "fuselage.png" file, which sits in the main "objects" folder. Only files with identical names will be recognized as "stand-ins" for the default graphic files.
So it is really important to respect file names and folder structures when creating your own repaints.
All of Carenado's planes contain a blank paint scheme, which is actually the default paint scheme of the plane. The blank files are located inside the main "objects" folder, and if you were to delete the "liveries" folder altogether, these textures would be the ones that get loaded. You'd have simply a white plane. These graphic files would be the starting point for your repaint.
To create your own repaint, start by creating a folder inside the "liveries" folder, called "MyRepaint". Then create a sub-folder, and call it "objects". Next, go to your aircraft's main "objects" folder (in your plane's top directory), and select all the files that need to be repainted in order to create your livery. Typically, this would include the fuselage, stabilizers, and wings, and maybe their night textures, depending on if the night versions require something different from the default textures.
Copy these files into your newly created "liveries>MyRepaint>objects" folder.
At this point, you're ready to start repainting those freshly copied files in Photoshop or GIMP.
Again, note that ONLY THOSE FILES that are inside your "liveries>MyRepaint>objects" folder will act as stand-ins for the default ones. So, if your repaint has completely white wings, you can simply use the default texture for that, and you wouldn't even have to include a "wings" texture file inside your "liveries>MyRepaint>objects" folder.
Only copy the files into that folder that need to be repainted.
We encourage users to express themselves creatively, using our included blank paint schemes as a canvas for their ideas. These liveries can be published to online forums, but ONLY INCLUDE THOSE FILES WHICH YOU HAVE ALTERED. Everything else is strictly copy written, and its distribution is not allowed. If you wish to publish a livery you have created, please create a .zip file of ONLY the folder that bears your livery's name, and publish that.
Carenado's planes include copyright notices. Please make sure you read and understand these, before publishing anything online. Re-publishing copywritten material without due permission is a criminal offence.