Just as with online tools, we have several options that we can choose from to compile our Sass files down to CSS files using offline tools. I've used Koala because it's easy to use, is cross-platform, and is free. You can download Koala for free from the project's website: http://koala-app.com/.

Koala gives you the ability to work with more than just Sass. You can also use Koala to compile and/or compress Less, JavaScript, CoffeeScript, and you can even use the Compass framework with it.

The best way to learn how to use Koala is to read the official documentation, which can be found at https://github.com/oklai/koala/wiki#docs. But if you'll just be using Koala to compile your Sass files (at least for now), let me quickly outline the steps for you here to save you from needing to jump back and forth between the book and the online docs. 

All you need to do is to create a web project using any text editor of your choice—such as Sublime Text or Visual Studio Code—and create a CSS folder as well as a Sass folder in your project's root folder. You don't need a completed project, of course—all you need is the very basic folder structure. Once the project structure is created, you can open Koala to get started, using it to compile your Sass files for you. Here are the basic steps:

1. Create an empty project structure, which, as a minimum, has the following:
   • A root folder with an empty index.html page in it
   • A CSS folder in the root folder with an empty styles.css file in it
   • A Sass folder in the root folder with an empty style .scss file in it
2. Open the Koala application.
3. Click the large plus sign (+) in the top-left corner, navigate to your project's root folder, and select it. At this point, Koala will find your styles.scss and styles.css files. 
4. Right-click on your styles.scss file in Koala's right-hand pane, select Set Output Path, and then navigate to, and select, your styles.css file in your file explorer 

Observing the preceding steps is all you need to do to set Koala up to compile your Sass files for you. The output of the compilation will be inserted into your styles.css file.