Summarize this article:

Click or double-click the QuickTime app icon, which resembles a blue "Q". If QuickTime isn't in your Mac's Dock, you'll find it in the Applications folder. This is in the upper-left side of the screen. A drop-down menu will appear. It's in the File drop-down menu. The recording bar will appear. This button is a circular red and silver option in the middle of the recording bar. Doing so indicates that you want to record the entire screen and begins the screen recording. Go through the on-screen steps you want to record. It's the square in the upper-right side of the screen. Doing so stops your screen recording. Click File, click Save..., enter a name for your recording, select a save location, and click Save. Your screen recording will be saved as a video file in your selected location.

Summary:
Open QuickTime. Click File. Click New Screen Recording. Click the "Record" button. Click anywhere on the screen. Perform your screencast's steps. Click the "Stop" menu item. Save your screen recording.