I use Adobe Photoshop 7. A few people here use GIMP which is a free program designed to be like Photoshop.
To get started on a skin you need to do the following:
-Make sure you have at least one version of the game set at 1024 textures with the Hi-Res pack installed
-Select the plane that you wish to skin in the hangar, right click and select 'save default skin'
-Go to your AH directory, go to 'skins' and open the folder that has been created of the aircraft you wish to skin. Eg '109_k41' or whatever it happens to be.
- There will be an image of the aircraft flattened out, for the 109K it is 109K41
- Open this image in your photo-editing program. It's also a good idea to expand the canvas size to 1500 x 1500 or so so that you've got a blank border around the template so that you can zoom right in on the very edges of it. I paint a couple of samples of the colours I'm using in the blank spaces there so that I can change colours easily, too.
- From here it's a process of 'tracing' over the panel lines and rivets (use seperate layers for these) and when you've finished that, reducing the opacity (to something like 10%)
- It's a good idea to colour in the surfaces of the aircraft in a base colour after you've done the rivets and panel lines. I use the underside camo colour (RLM 76 blue, for example).
- From there you can start doing the actual camo... then the dirt, exhaust stains and paint chips.
- Be careful to keep things on seperate layers so that you can adjust things more easily. I find that colour-coding the layers in the layer tab a helpful trick to make it easy to identify which layer is what. (dirt, camo, rivets etc)
You will start with this (slightly edited on the left fuselage):
http://i74.photobucket.com/albums/i253/plague_06/Aces%20High/109K41.jpgand end up with something like this:
http://i74.photobucket.com/albums/i253/plague_06/Aces%20High/109K4SkinJG3DeskauMain.jpgTo view your progress you will need to download a plug-in called 'bright'. You export the image through the bright plug-in and then replace the standard template with your edited one (eg. rename it '109K41' for the 109K) and then load up the game and your skin will replace one of the options available in the skin drop-down bar in the hangar.
That should get you started.