Here's how I do it since using the compiler is too much effort for me
Things you will need: Notepad++, paint.net/photoshop/gimp/whatever
1: Create a new folder for your reskinned part/parts (I like to add 0s or -s as the first letter in the file name so it comes up first in the components list)
2: Grab the .gmf and map of the part you want to skin and put them in your new folder
3: Open your map (it's either going to be a .bmp or .tga) with your image editor of choice, since you will probably want more detail in the part and most parts have standard maps that are 256x256, 128x128, or even 64x64 I suggest the first thing to do after you open it is resize it to 512x512.
4: Put whatever you want on the component (It's optional to use layers, but I think they help a lot when putting multiple things on one parts)
5 (Optional): Save a .pdn/whatever photoshop uses/whatever gimp uses of your skin first incase you want to tweak the layers later as .bmps don't support layers, give it some cool name I guess.
6: Save your part as a 24 bit .bmp or 32 bit .tga (both uncompressed)
7: Go to your maps folder and rename your new .bmp/.tga to something with the
same number of characters as the old .bmp/.tga, this is important and if the number of characters isn't the same it won't work. Since the original .bmp name was "snowplow1" I can put anything I want as long as its 9 characters, so I will use "memeplow1"
8: Go back to you main folder for the reskinned part (In this case, 0dope), rename your .gmf to match your .bmp/.tga (so memeplow1.gmf) and open it with Notepad++
9: Go to Search, then find and type in .bmp/.tga in and find where the map is specified in the .gmf
10: Change the file name specified to the .bmp we made earlier, so again, memeplow1.bmp and then save the file
11: Go to your main components folder and locate the .txt file for the parts
And make a copy of it and rename it (I like to name it the same as the folder I created in step 1)
12: Open the .txt file, change the dir to the folder we made in Step 1, the .gmf to the one we made in Step 8, and save the file. (Optional but recommended, remove the styles from the .txt file)
13: Go open RA2 and see if you did it right, if you did you should have a new reskinned part