This is a pretty big subject but I'll give it a go.
First of all you need to take a look at the existing AH2 skin. Right click on the plane you want to skin in the AH2 Hangar and save it. This creates a subdirectory in the skins folder containing all the art files for that aircraft. Most of these files are instruments etc. and can be deleted. The external art is in two 1024/1024 res 256 colour files.
Open these in PSP. You first need to determine whether you can actually achieve your scheme. AH art often has limitations and occasionally bugs which makes it impossible to get some schemes to work. Sometimes one piece of art does double duty for more than one part of the aircraft, i.e. left wing panel art is flipped over to double as the right panel. Another common problem is where pieces of art are stretched to infinity across parts of the airframe. This is usually the top and bottom edges of the fuselage or around the cowling.
Once you have found a suitable scheme create a new folder in the skins folder. For a P51D you would create a p51d1 folder to go with the original p51d folder. Copy the files you are going to modify in the new folder. Open these in PSP and increase the colour depth to 16 million colours. Most of the neat tricks PSP can do only work with this colour depth. You may also want to increase the resolution of the image to 2048/2048 (image/resize). This improves quality but makes the finished psp files much larger. I made the mistake of going to 3072/3072 and ended up with files of almost a gigabyte each, very sluggish. I'd think 2048 should be OK if you have a 512mb or ram or more and a modern PC. Once you have done this save the new files as a psp image.
One of the main reasons for going to 16 million colours is it enables layers. Layers can be thought of as invisible sheets of tracing paper. You draw/paint on each layer and lay one on top of the other to create the finished image. PSP lets you reorder layers, make them more or less transparent or completely invisible. They also make editing much more simple. You can move a hatch on one layer a few inches without it effecting the paintwork underneath or the oil streak running across it.
What I do is create the panel line, fastener and rivet layers first. These usually need a lot of mucking about until they fit properly and everything else (colour scheme, weathering etc.) needs to fit round them. One thing I have found is that it is also better to have seperate layers for horizontal and vertical lines. This makes them easier to edit.
To start with your image will have only one layer labelled "background", this is the original AH2 art. To create a new layer first make sure the layer pallette is toggled to "on" and click on the "create layer" icon in the layer pallette. Label it something like "panel lines, horizontal". Once you get to 40 layers or so it can get hard to find stuff, so think hard about how you are going to label all the layers.
Once you have the panel/rivet layers created you can start drawing your lines etc. I've found it best to stick to one layer as much as possible when working. It is very easy to do a load of work on the wrong layer. While this often just means copying and pasting or relabelling the layer, sometimes it can ruin hours of work.
A couple of tricks you can use while editing panel lines. First, make all the lines a bright colour to start with. You can always use the "colour replacer" tool to turn them back to black later. This makes any line mismatches between different panels easier to spot, particularly if you create a temporary plain coloured layer beneath all the lines. Another technique is to create a copy of each panel or rivet line layer and save it as a new layer on top of the first. Turn this layer's lines to white with the colour replacer then offset it a pixel or two up or down and to one side. This gives the lines an edge where the light is glinting off them. Use the layer transparency sliders to adjust the brightness of these layers. Also think about keeping the relative lighting angle consistent between layers if you do this. Take a close look at some of the default AH2 art to see this technique.
Once you have got to a stage where you want to see what your skin looks like in AH2, save each file, then do a "save as". Save the images as a bmps into the "p51d1" (or whatever) folder. Open these bmps (not the psps) and resize them to 1024/1024, then reduce the colours to 256. For the reduce colours options I use "optimised octree", "error diffusion" and "reduce colour bleeding".
One other thing before you can view them, create a text file with notepad called "p51d1.txt" or whatever your plane is. This should have a 20 or less character description of the skin. You will see this in the Hangar's drop down menu in the game. Save the text file in the same folder as the bmps.
Start up the game and select your art from the drop down menu in the hangar. Fly your plane and take loads of closeup screenshots from all angles. Exit the game and view the screenshots. Pay particular attention to stretched texture areas and where panels come together, i.e. wingroots. Make a list of adjustments then reedit your psps. Repeat as necessary.
Once you have done the panel lines etc, start on the colour scheme, then do the weathering etc. You need to think about exact order of the layers, but usually it is scheme on the bottom, then weathering, then panel lines on top. Some pieces like radial engines can be created as seperate images and pasted in when completed. This cuts down on the number of layers. Good luck.
