Detailed information about Xresources configurations can be found in the Arch Linux Wiki and Wikipedia.
Copy the content of the everforest file into the ~/.Xresources or ~/.Xdefaults file and reload the settings with xrdb.
Copy the everforest file to any place and import it via #include "/path/to/everforest".
To merge the color theme into your current settings copy the everforest file to any place and run xrdb -merge path/to/everforest.
Based off sainnhe's Everforest.