Navigate to your gemfile and add gem 'materialize-sass' in the general scope of the Gemfile.
Run bundle install from the terminal within your project directory. This will install the Materialize-UI gem, which includes the javascript and scss files needed to incorporate this into your app.
Edit: Additional Steps Required:
Add @import "materialize"; to your app/assets/stylesheets/application.scss file.
Add //= materialize at the end of your app/assets/javascripts/application.coffee file.
Add //= materialize-sprockets at the end of your app/assets/javascripts/application.coffee file.
Run rails assets:precompile
Restart/start running your project with rails s to ensure the changes are made.
Enjoy your new Materialized project!