Open up your downloaded marks (it commonly comes in .zip file or .rar file), and you'll see a lot of .tga files. You can view them with paint or PS.
Go to your CF folder, for a shortcut, you can right click CF icon and click "open file location".
Go to rez, UI, then Marks folder. Inside is all your kill marks you can edit manually yourself too.
Drag all of the contents inside your .zip/.rar file, (the file you downloaded, PUT ONLY THE CONTENTS INSIDE), to your CF Marks folder.
It WILL tell you if you want to overwrite. There will be many to overwrite, so at the bottom you'll see "do this for all conflicts" with a checkbox beside it. Check it, and click "move and replace". This will move and replace all of the new marks with the old one you have already.
If it doesn't ask you for overwrite, that means the contents inside your ZIP / RAR file is NOT in .tga file. Those type of files need to be arranged to .tga file by using a program. Usually when kill marks do NOT come in .tga file, they will come in .jpg files. If they do come in .jpg files, use this source: Free JPG to TGA online converter | Image Converter Plus | How to convert JPG to TGA online
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