How I installed it.
Download Map Editor from gta5-mods.com. Use it's supplied "Custom_map" dlc. make sure to add it to your dlclist.xml
Drag & drop all files EXCEPT the paths folder to:
mods/update/x64/dlcpacks/custom_maps/dlc.rpf/x64/level/gta5/_citye/maps/custom_maps/ folder.
Add all the paths in:
mods/update/update.rpf/x64/levels/gta5/paths.rpf/