Disclaimer: I'm not actively updating these so psyonix could make changes to their files making some of these outdated. If something's not working, do it manually using my step-by-step guide

Reddit thread

Extract the files into UDK/Custom/UDKGame/Content/

Open UDK

Right click on UDKGame and press 'Fully Load'. First time doing this will take a while, but after that it's going to be almost instant.