Once the zip file has downloaded, you need to locate it and then unzip/extract it.
It's not important where you unzip/extract it to as long as you remember where you put it.
The next stage is to go into Chrome's Extension management, which you can get to by clicking on the Customize button (the three horizontal bars stacked on top of each other), then click Settings and then on the left hand side of the settings page, click Extensions. If you want to use the keyboard to get there, type chrome://extensions into the address bar.
Once in the Extension page there is a checkbox at the top right called Developer mode, you must check this box to be able to manually add an extension.
After you check that box you will see three buttons, click the left-most button that says Load unpacked extension...
A file dialog will appear prompting you to Browse For Folder, locate the folder you unzipped/extracted the extension to.
If you didn't change anything during unzipping this folder will be called GuardianCirfer.
Click OK and the extension will be added to Chrome, you should see the CiRF the Guardian 'G' logo at the top left of the Chrome window.
If the 'G' logo doesn't appear ensure the Enabled checkbox is checked.
If you use Chrome's Incognito mode you can check the Allow in incognito checkbox (please note the warning about "extensions recording your browsing history", but we don't do that anyway).
You can now uncheck the Developer mode checkbox and close the Extension tab.