To get started, the first thing you'll want to do is download the Chrome extension:
Once the extension has downloaded, paste chrome://extensions/ into your URL bar to load a page that lists all your extensions. Drag the src.crx file that you just downloaded into the extensions page, click the "Add Extension" button when it asks whether you want to add the extension, and you're all set!
If you're a developer and you might want to modify how Helena works, you may prefer to follow the installation instructions for developers.
Once you've installed the Chrome extension, here are some resources for learning how to use it: