Instructions from https://help.screencastify.com/article/224-how-to-make-your-first-recording 

How to make your first recording

Once you've installed and set up Screencastify, you're ready to create your first recording. 

Starting a recording

1. Click the Screencastify extension icon (if you can't see the icon, click here)
2. Choose what you want to record: your browser tab, entire desktop, or webcam
3. To narrate over your recording using an internal or external microphone, turn on the toggle next to Microphone
4. To capture the audio originating from within your browser tab or application (e.g. from a video being played), click "Show more options" and turn on Tab Audio or System Audio
5. Turn on the toggle next to Embed Webcam if you want to include your webcam as part of your recording
6. Click RECORD. You'll see and hear a countdown, and then your recording will begin.

See attached on video on how to start video and select sound on tab.

NOTE

When Screencastify is recording, a red dot ( ) will appear in your extension icon.

Ending a recording

To end a recording, click the extension icon again (or use a keyboard shortcut) and click the stop button. 

See attached on video on how to stop video.

TIP

You can also pause and resume a recording.