Unpack the files into one folder, which contains a single folder for every desired livery you want to use.
Like
Mercedes-DTM/Livery1/**dds-files in it**
Mercedes-DTM/Livery2/**dds-files in it**
etc.
Then, you need Crow's tool.. Select the DTM car (mercedes_c_touring_13 or something like that), select and load the folder with all the liveries (in this example "Mercedes-DTM").. You can now see Livery 1, Livery 2, etc. in the GUI.. Select them, move them into the livery list (>>) and assign the grid.. You can now export the package and call it whatever you want to, let's say "DTM pack".
You can now close the tool and have to open JGSME (or something like that, not in front of my system right now) from the pCars folder.. Activate a) Custom_liveries_tool for all liveries assigned, not only a specific car, or b) the "DTM pack"..
Start pCars now and you should be able to select the new liveries..
Like I said, I'm not in front of my system, so the names are just examples, but I hope the procedure is clearer now..
To avoid crashes of the tool, make sure there are only the livery files (*.dds) inside the folders, not the zips nor renamed files..
