Example: E170
1. check if a livery exists:
http://liveries.flightgear.org/airlines.php?id=29 -> Nope, does not
2. get the paintkit, either from liveries.flightgear.org or the aircraft repo:
https://github.com/FGMEMBERS/E-jet-fami ... ter/Models (You see paintkit170.xcf? That's it!)
3. get GIMP (there are other, but IMO that's the best program:
http://www.gimp.org/4. Open the paintkit in gimp:
5. press CTRL+l to enable layer dialog. This let's you (de-)select layers.
6. Create new layer: Menu -> Layers -> New layer
7. Start with painting:
Note1: I know this isn't realistic or looks well, it's just for demonstration.
Note2: I can't teach you about all the gimp basics. That's too much for here but you can find huge resources on the internet using google (or any other search machine), your friends
Note3: If you are at this step and interested, I'll continue.