Enabling offline mode on chrome is quiet easy. I don't know what your intention is with chrome's offline mode, so am going to give a brief description of how the offline mode works.
What the offline mode does is that, chrome will create a local cache of all your previously visited webpages. So if you visit any of those web pages again without an internet connection, chrome will automatically load a previous copy of the webpages from the saved local cache.
To enable the offline mode, you'd need to open your chrome browser as usual. Click on the options menu from the top right corner of your home page. Click on settings and search for the option of offline mode. You can then enable it. When enabled, every webpage you visit, a local cache will be created for them and pages saved.
To use the offline mode when there's no Internet connection, all you need do is type the address as you'd normally do when browsing. Since there's no internet connection, you'll get a no internet connection message on the browser. Below the error message will be a blue highlighted option of browsing offline. Simply click on it and you're good to go.
Remember, offline mode does not enable you to browse webpages that hasn't been previously accessed without an internet connection. This simply means that you can't visit new site that doesn't have a previously saved local cache.