Ok...first of all, you're going to need an existing F7 from JR. After that, you'll want to clone it and open it for edit in explorer. When you finish your skin, you'll save it as a map1.tga there.
In GIMP, go to the windows tab and go down until you see the Layers, Channels...etc tab. Click on it. This will show you all of the layers at work in the .psd, as they have quite helpfully split everything up into layers for you. Once you made it this far, I think you should be able to figure it out on your own. Their maps are somewhat complex but after a while you get used to what goes where, and everything has its own mapping so you don't have to worry about your plow also being mapped to your sunshades and half the radiator section. Just take your time and you should be fine.
One final note, when you are exporting the finished skin to tga, it will ask you if you want to use RLE compression on it. Trainz cannot open compressed tga's, so be sure to uncheck the box before saving. Once you've exported, go to the config and change the name of the asset to match what you've made so you can find it under something other than new asset. Once you've saved that, go back to trainz CM, right click on the asset and submit edits. This will save your work, and after this you will be able to view it in game.
One invaluable piece of software that I use is PEV's mesh viewer. You'll have to export to tga every time you want to see what you've done, but it is extremely helpful in allowing you to see what you've done without having to save everything and look at it in game, and if you want to line up striping or something along those lines, that will help you make minute adjustments should they be required. Here's a quick render to show you what I'm talking about, right now I'm using it to check how my weathering looks on this GP15 that I am presently working on:
(sorry JR for non JR image, trying to give an example, can be removed if necessary)
A couple of final tips, don't make your colors too saturated, they will almost always come out brighter in game. Also, total black and total white are too dark and blindingly bright in game respectively, I use R:22 G:22 B:22 for black and R:235 G:235 B:235 for white, although everyone's preferences are a little different.
Anyways, good luck!
