First you need to download Lambda's map editor:
https://www.mod-rdr.com/downloads/rdr2/mods/67-red-dead-redemption-2-map-editor/

*Important* - Currently this map only works with version 0.1.6.1 and lower. Future versions may require a map conversion tool. Lambda is currently working on that.

Put the map into 'Rockstar Games\Red Dead Redemption 2\MapEditor\Maps' folder