Apple pre-installed QuickTime Player, an app that lets you record, edit, play, and share audio and video files, onto every Mac. You can use it to record your computer screen.
1. Open QuickTime Player by searching for it in Launchpad.
Launchpad
2. Once you're in Quicktime, click “File” and select “New Screen Recording”.
3. Press the downward facing arrow next to the “Record” button to choose the following recording options:
Microphone: Choose “None” to record your screen with no sound. Choose “Built-in Microphone” to record your screen with sound.
Options: Choose “Show Mouse Clicks in Recording” if you want your clicks to appear as a circle during your screen recording.
4. Click the “Record” button and choose one of the following recording types:
Click your screen to record your entire computer screen.
Drag a section of your screen that you want to record.
5. To finish recording, click the “Stop” button in the menu bar, which will be to the left of the “WiFi” button.
6. To save your recording, click “File”, press “Save”, and name your recording.
If you wish you can save from youtube some examples of how this is executed.