Right let me start my bit off by saying I'm no expert, there maybe easier and more time efficient ways to acheive the finished skin you want.
But this is my basic start to creating a skin.
I use both CS5 and GIMP to do my editing both have their strengths and weaknesses so I have the best of both worlds.
So, I start with three layers, the main skin, panel line layer and lastly the weathering layer, now depending on which aircraft you want to skin not all of them are available from your skins folders.
You can use the fantastic Kegetys mod available here:
http://www.kegetys.net/
To extract out the various layers that go to make up these skins and then use a programme called DXTbmp to convert them from .dds file to 24bit bmp files. ( I wont be going into setup details RTFM

).
DXTbmp here:
http://www.softpedia.com/progDownloa...oad-53836.html
If you have set up the Kegetys.dll correctly using the _Dump command you will end up within a folder within your Cod folder wherever it may be, mine is in : C:\Program Files\Steam\steamapps\common\_dump, you need to run the game for this folder to be created.
The files you need are in C:\Program Files\Steam\steamapps\common\_dump
\$bob\3do\Plane
The files I use are:
1. M-diffuse0.dds (main skin)
2. M-overlay0.dds (panel lines)
3. W-diffuse0.dds (Weathering)
Now to convert them to 24bit bmp's, ( I use BMP becuase I dont want to introduce jpeg noise at this level) using DXTbmp you just drag and drop them into the programme and it will display the file, quite simply save it as a 24bit bmp giving it a name and saving it to a place that works for you, repeat with the other files and your ready to go with your three basic layers.
Asleep yet?
In Gimp/CS5 I load up the three layers into one project, I start with the main skin switching the panel and weathering layers off for the time being, sampling each colour I flood fill them back using a tollerance that will only fill back the colour Ive picked, this cleans them up, of course you can change the colour completley depending what your trying to achieve.
I then go around all the edges where colours meet to clean them up, the line tool comes in very handy, especially for Luftwaffe Camo, just make sure you whichever tool you use have anti-alias on otherwise you will end up with very jaggy lines and it just looks pants.
Now I switch on the other two layers
the order is from top to bottom:
1. (Main skin layer)
2. (Panel lines layer)
3. (Weathering layer)
At this stage.
My Main skin layer has an opacity of 80%
My Panel lines layer has an opacity of 90%
And my Weathing layer is set to 0%
Once your at this stage your ready to paint, you can see clearly where most of the panels are just make sure you paint on the right layer.
3----2-----1---- your back in the room!
Thats my basics....