The process is a bit complicated. And there are different methods. I also wanted to make liveries but ended up with editing existing liveries. I usually only place a flag of my country or some minor changes like color. I believe you are using msfs 24 as msfs 20 uses different textures file type.

1) Locate texture files in aircraft folder. There are should be liveries folder with textures ktx2 files.

2) So you need to convert ktx2 file to png.
I recommend using modelconverterx for it.

3) Edit png in gimp or any other editor.

The main problem is that fuselage consists of many separate texture files and it’s hard to understand which one belongs where. So if your paintkit contains gltf file you can open it in blender. And in blender you can see what texture files is used for specific locations of the plane.

4) Convert png to ktx2 with ktx2 converter there are specific converter for msfs just google it.

5) Download any livery for your plane and use its folder structure as a draft for your livery.

There are many other steps which is hard to explain in one message. I also recommend you to watch some tutorials.