It's a very easy thing to do my friend
First of all you should have the textures for the car you want to paint, and in that situation, you have 3 solutions:
-if it's a default car (the one's that come with the game) you gotta get DeAR2 to extract the files from mm2tex.ar.
-if it's a custom vehicle you got from mm2x or another site, you have to rename it's file extension from .ar to .zip. Then, you just gotta open the file by double clicking it if you have WinRAR or a similar software installed in your PC. However, two things can happen now.
--If it throws you an error, saying it's corrupted, it's most likely locked, and as far as I know, there's no way to open them, so no dice there.
--However, if it opens as it should, you should be presented with some folders.
Now, create a folder and name it whatever you want, and inside that folder create another one called "texture". This is where you'll put your custom paintjobs.
This is is the same for default and custom cars, in which you have to search in the "texture" folder, and inside you should have some files, them normally being either .BMP, .JPG, .TGA or some others, MM2 supports some file formats. Anyway, you gotta pick the one you want to modify, and copy it to the folder you previously created.
Then, you just gotta open it with the image editing software you want! I mean, I think even Paint would do, but you better use some more professional solutions, such as Photoshop, GIMP or Paint.Net.
Now, I'm not gonna teach you how to modify the textures, you can find those tutorials in Google
Finally, after the modifications have been done, save the texture as the same file format it was originally, and save it to the "texture" folder you created, with the same name as the original one.
Finally, you gotta go back to the root folder where your "texture" folder is located, and zip it, preferably without any compression, and name it "! ! ! somethinghere.ar".
Finally place it in the MM2 folder, and you're done!
It might look like a difficult process, but once you've done it once or twice, you'll realize it's really easy