For some reason, the Susy website doesn’t make it abundantly clear how to install Susy.

I’m going to go ahead assume you have Node / Ruby / Sass installed and Compass. Suzy requires Compass version 1.x.x or later (as it is part of compass). 

1. Check your compass version 

From the Terminal check your compass version:

compass version

It should return something that looks like this:

Compass 0.12.7 (Alnilam)
Copyright (c) 2008-2014 Chris Eppstein
Released under the MIT License.

Note: Running the compass installer as of writing this will install a 0.x release. You will need to install the prerelease version. If you see Compass 1.0.0.alpha.x  or later, skip step 2!.

2) Install Compass prerelease

From the terminal run the following command:

sudo gem install compass --pre

This may take a few minutes and you’ll see a lot of install in your terminal

3) Install Susy!

Now we’re finally ready for susy. From the terminal run:

gem install susy

Congrats! You should be in business. Note you’ll need to install Node.js, which can be downloaded here, Ruby which which can be downloaded here (OS X comes with it preinstalled), Sass which instructions can be found on how to install here, and you’ll need to set up a compiler like Grunt.js or Gulp.js or an application like CodeKit or Prepros to run compass to compile your SASS.

Happy Coding!