1) Subscribe to the mod then launch L4D2 to the main menu.
2) Turn off the game when it's done loading the mod.
3) Go to your C:/Local Disk/Program Files (x86)/Steam/SteamApps/common/left 4 dead 2/left4dead2/addons/workshop
4) There is a VPK file with the same number of the picture of Re:Zero Kara Hajimeru Isekai Seikatsu
5) Open it with GCFScape and extract the file "READ ME"
6) Follow the instruccions in "READ ME"!