SASS is an extension of CSS3, adding nested rules, variables, mixins, selector inheritance, and more. It's translated to well-formatted, standard CSS using the command line tool or a web-framework plugin
First install ruby before installing SASS. SASS is built upon Ruby. If you are working on a MAC, chances are, you already have Ruby installed. If you may to install Ruby in Windows, use this Ruby Installer.
1) Install Ruby from http://rubyinstaller.org/
After the installation is complete, you can go to Terminal (on a Mac) or in Command Prompt (on Windows) then type the following command line in it:
On Mac;
sudo gem install sass
On Windows;
Change directory to Ruby bin folder. eg C:\Ruby193\bin
gem install sass
SASS watch
On Mac;
Terminal change directory to project folder.
sass --watch sass:css
( sass and css are difference folder for .css and .scss files )
On Windows;
Change directory to Ruby bin folder. eg C:\Ruby193\bin
sass --watch path/sass-directory:path/css-directory