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Doing so takes you to the app's camera screen. Your video can only be 10 seconds long, so choose a subject that can be captured in a short clip. Tap the button with two arrows in the upper-right corner of the screen to switch between the front-facing and rear-facing lenses.  The front-facing camera works best for selfies because you can see yourself on the screen as you shoot the video. Tap and hold anywhere on the screen to activate Snapchat Lenses. Lenses use face-recognition technology to add effects, like dog ears, to people. Scroll left across the capture button and follow any onscreen prompts to see what the lenses do. It's the larger circle at the bottom center of the screen. As you hold the button, the white outer circle will turn red, indicating the duration of the video, and a solid red circle will appear in the center of the capture button to show that your camera is recording. Doing so will stop the recording. Your video will stop automatically after 10 seconds, when the outer circle of the button is completely red.

Summary:
Open Snapchat. Select a subject for your video. Select a camera mode. Press and hold the capture button. Release the button.