1. Install HomeBrew following https://brew.sh/
  2. Install RVM according to these instructions https://rvm.io/rvm/install
  3. Install Ruby e.g. version 2.x.x
    rvm install 2.x.x
  4. Set Ruby version
    rvm use 2.x.x

    If that gives an error…

    /bin/bash --login
    rvm use 2.x.x
    
  5. Follow this guide https://usabilityetc.com/articles/ruby-on-mac-os-x-with-rvm/