The video on the right is a great introduction to using Screencastify. This presentation is another useful introduction. Make sure to set it to save to Google Drive. Go to Show Existing Screencasts and click the dropdown in the top left. Select Options and select Save to Google Drive. You will need to accept the permissions and may need to log into your Drive.
If you want to have a small video of yourself in the presentation select Embed Webcam and choose where you want the window to appear. If you want to just video yourself select Cam at the top.
Other Screencasting Options
Narrate Over Google Slides
Open your Google Slide presentation. Select Present with speaker notes. Minimise the speaker notes. Go to the screencastify icon and record as normal.