ERROR: While executing gem … (Gem::FilePermissionError)

ERROR: While executing gem … (Gem::FilePermissionError)
You don’t have write permissions for the /Library/Ruby/Gems/2.3.0 directory.

1. Install RVM (optional)
2. Set the gem folder in your shell resource file. I.e. .zshrc, .bashrc etc

export GEM_HOME="$HOME/.gem"

3. Install you gem gem i LIBRARY_NAME