To install this you need to have a dlc pack called custom_maps which can be downloaded here

https://www.gta5-mods.com/tools/mapeditor-2-ymap-converter

This is fully compatible with Forests of San Andreas and I also recomend installing it as it brings your game 
to life with additional trees and adds a cabin and some other cool stuff

https://www.gta5-mods.com/maps/forests-of-san-andreas-4861-trees-add-on-ymap

If any help is needed installing don't hesitate to ask in the comments.


For anyone who needs help installing the ymap

https://www.youtube.com/watch?v=Kt6Tm45kkwI