The first step is to download the extension from the google chrome webstore. In the webstore search option type in "Screencastify" your results page should look like this.
Once you add the extension it will appear at the top right of your chrome browser and look like a film strip. To use this extension simply click on the film strip and you will see a menu to start recording.
My favorite parts about using Screencastify is how easily you can create a screencast and upload it right onto your Google Drive. This is by far the greatest advantage to creating screencasts with Screencastify over other apps. Once a screencast has been recorded the student can now share it with the teacher. This really puts the power of creating work in the hands of each student and provides a simple way to make sure that the work will not be lost.
To use this in my lesson I had students add the extension on their chromebooks, then after typing up their assignment in Spanish, students used screencastify to record them speaking their paragraph. The students then shared their screencast with me so I could assess both there writing as well as their speaking. Screencasts make an oral and written assessment possible with all of the benefits of Google Drive. I can now open up their screencast in Google Drive and post a comment about their screencast for the student to see. The videos can then be downloaded and saved on a computer or imbedded into a website. Check out one of my students screencasts to see what a screencast can look like.