You can make Snaps even more fun by adding real-time special effects and sounds with Lenses! To use Lenses in Snapchat follow the steps below:
  1. Go to the Camera screen in Snapchat.
  2. If your rear-facing camera is active, switch to the Selfie camera.
  3. Press and hold on your face! Lens options will appear below.
  4. Swipe left to select the Lens you want to use.
  5. Follow any action prompts that appear, like ‘Raise Your Eyebrows.’
  6. Tap the capture button to take a Snap, or press and hold on the capture button to record a video.
Note: Lenses aren’t supported on some devices including older iPhones, iPod Touches, and many older Android devices. Also, recording video with Lenses is only supported on Android 5.0 or higher.

source:support.snapchat.com