1. Install Google Chrome if you have not installed yet.
2. Visit google.com
3. search for Screencastify extension for Chrome.
4. click on screencastify first option as seen in the above picture
5. Now click on the Add To Chrome blue button to install this extension.
6. Once this extension is installed you will see a well come page.
Now click on the get started button
Once the extension is installed you can select wheather you want to save this video in your local computer or you want to save it on google drive.
you will see the black logo on top right of your chrome. Click the button to view and start the recording.
You can select weather you want to record your tab only or you want to record the whole desktop.