There's not an easy way to tell someone how to re-skin a locomotive. I use Adobe Illustrator and Adobe Photoshop. This is my method and it works quite well for me, however, I'm sure there are many other ways to re-skin.
 
First, you need to make sure that you aren't planning on reskinning someone else's work with evil thoughts of SELLING "your creation." Every locomotive I have reskinned, I did for my OWN USE. You also should check the Config file to see the original creator's messages. Some say, "No Reskins Permitted." Other's say "Email me for permission to re-skin." Common courtesy there.
 
If you find one that says something like "Free to reskin" then there's no problem, as long as you're not planning on 
reskinning someone else's work with evil thoughts of SELLING "your creation."
 
The first thing you have to do is copy the loco files that have the art of the side, front, rear, top, tanks, accessories, and anything else that will need to be "repainted." You can't just go into your paint program and start coloring away, or you'll be "painting" over the logos, door/hatch line art, radiator vents, etc. etc.... 
 
How 
I do it:
 
First, I use Adobe Illustrator to re-create the locomotive art. I load up one of the files that I'll need to re-work the line art, color, logos, etc. Using layers, I first re-draw the line art--- i.e. the outlines of engine doors, hatches, vents, bolt heads, and a hundred other things. But you can't just draw black lines. You must take care to draw the line art with varying shades so as to represent the shadows, rounded areas, bulges, etc. etc. (My method is very VERY time consuming, but I am pleased with my final results. I had many years of experience with Illustrator and Photoshop because I was a technical illustrator and graphic artist before I retired.)
 
After I am finished and satisfied with my line work, I add the coloring under (normally) the line art. Next I add logos and loco numbers and lettering. I save each file under the same name as the original file. It will save it as an AI file (Adobe Illustrator). When I have done all this, my next step in to use Photoshop to actually re-skin the locomotive.
 
I find the locomotive I am planning on reskinning in CMP. I CLONE it, then open for edit. I find the first file (for instance the SIDE of the locomotive) and open it in Photoshop. I go to my Illustrator file and open it in Photoshop also. It should be EXACTLY THE SAME SIZE as the file I am reskinning. I simply copy my Illustrator file and paste it over the old file.
 
If you want, you can add weathering next. Then, flatten the image and save. When you have changed all the files with your Illustrator files, open a map and place the locomotive on the tracks. You'll probably see a dozen little things you'll need to change, such as lines that don't line up correctly, backward stuff, etc. etc. You'll need to take screenshots of your "mistakes" so you can correct them in Illustrator. Then, when you've corrected these mistakes, you'll need to copy and paste your Illustrator files again.
 
This is just one way of reskinning. Like I said, this is the way I do it. There are probably lots of others. Maybe even some a lot easier and better than my method. But, this is the way I taught myself. It works for me.
