Jade is billed as a template engine for Node.js, but I think it's better thought of as a language that makes HTML easier to write and also lets you do templating.

To use Jade, you must have Node.js installed on your system. That's better convered elsewhere, so I will not cover that here.

Once you have Node.js installed, you have the choice of installing it so it available system-wide or only for a particular project. To make it available system-wide, open a terminal and enter

npm install jade -g

Until I get a good grasp of using Jade, I want to install it on a per-project basis. To do this, open a terminal in your project directory (or navigate to the directory) and enter

npm install jade

When this is done you should see a node_modules subdirectory. Inside that directory will be the Jade modules as well as any other modules Jade depends on.