Guys here is my process it’s a quick version as I am on the bus to work ;-)
I open up the mas file from the latest version of the car you are skinning it’s the file in the most recent folder it should have veh files dds files and an upgrade ini file in it. It’s the only one you can open usually as 397 encrypt the others
I take the any veh file for the car I am skinning and place it in a folder I usually name this folder build but the name is not important it’s just a place to collect the files.
I then rename the veh file this is important as duplicate veh names are ignored by the game so you car won’t show if you keep the same name. I use race_yy_nn eg B12_19_25 you then can open the veh file in text editor and change driver names class categories etc the most important is to change the skin file reference at the top make it the name of your skin I use the same name convention for skin as veh file. I like this way of skinning because it allows you to set up categories and classes. I would also change the description to match your skin. You can look at my mas files for examples
Then spend countless hours making your skin

once ready add the skin dds and the other dds you have to the build folder with the veh file in it and create the mas file this file needs to be placed into the most recent folder of the car you are skinning. The only messy part is if 397 updates the car you need to move the mas to the new most recent folder. I give the mas file a meaningful name so you can search for it if required or when you look at it you know what it is this is important when you have a few mas files in the one folder
Tools you need is only the rf2 mas tool you can find it in the rf2 install, sorry I am not near my pc so I can tell you the location. You use this tool to extract the veh file originally and to pack the final skin mas.
hope this helps a little, please pm if you have any questions I am happy to help if I can
