Once you have acquired and installed the Narratives for Tableau extension from the Chrome Store, you will need to register in order to start generating narratives from your Tableau sheets and dashboards. From a Tableau sheet or dashboard, once you open the extension for the first time, you will be prompted to register.
Once you fill in all the details, the orange "Register" button will be highlighted
Click register and you will be taken to the selection of measures and dimensions for your first narrative. From this point forward, you will not be prompted to register again unless you switch browsers.