Here's a copy and paste for terminal to set this up:
# Be sure to change:
# FOLDER_NAME - the root folder name.
# ACTUAL_PATH_TO_REAL_FOLDER - the path to the "real" folder.
#
# Example:
# "Drives Users/bob/Documents/Drives"
# .......^ this is tab (not spaces)
# Both of FOLDER_NAME and ACTUAL_PATH_TO_REAL_FOLDER are not starting with /
sudo touch /etc/synthetic.conf
sudo chmod 0777 /etc/synthetic.conf
sudo echo "FOLDER_NAME ACTUAL_PATH_TO_REAL_FOLDER" >> /etc/synthetic.conf
sudo chmod 0644 /etc/synthetic.conf
sudo chown root:wheel /etc/synthetic.conf
