Sage isn’t really meant to be used as a one-time install. It’s a base or starter theme to code your own theme on. Along with that, you do need a relatively good understanding of the command line and be able to install Node.js, gulp.js, and Bower on your own, as well as being able to troubleshoot a bit on your own.
That being said, the instructions on how to get Sage working are in the readme: https://github.com/roots/sage#installation
If you are having trouble with Node.js, this post is stickied: Npm Debugging Guidelines (Failed npm install, bower install, or gulp build? Read this)
Finally, and I don’t mean to be rude, but Sage is not a beginner’s WP theme. Before jumping into Sage, you should really have a very good understand of how a WordPress theme works, and again, not be frightened of using the command line since that is how the workflow of Sage is being used. We’ve written this post to try to explain this: https://roots.io/sage-should-not-be-your-first-wordpress-theme/