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Go to https://handbrake.fr/downloads.php in your web browser and click the Download link below your computer's operating system, then install Handbrake by doing the following:   Windows - Double-click the Handbrake setup file, then follow the on-screen installation instructions.  Mac - Double-click the Handbrake DMG file, verify the download if necessary, drag the Handbrake icon onto the Applications folder shortcut, and follow any on-screen instructions. Its app icon resembles a pineapple next to a drink. The Handbrake window will open. It's in the upper-left corner of the Handbrake window. If this is your first time opening Handbrake, you may not need to click Open Source in order for the sources window to open. It's the folder-shaped icon in the Sources window. Go to the location of the video that you want to compress, click the video, and click Open. This will open the video in Handbrake. On the far-right side of the Handbrake window, click one of the quality and framerate presets (e.g., Very Fast 720p30) that matches your video.  Make sure that you select a preset that falls at or below your video's current quality. For example, if your video's current quality is 1080p, you'd click a 1080p option or lower; if the video is 720p, you'd click a 720p option or lower. The Fast and Very Fast options are best for compression. Replace the file name in the middle of the Handbrake page with a new file name (e.g., [video name] compressed). You can also select a new save location by clicking Browse, selecting a folder, entering a new file name if necessary, and clicking Save. This box is in the middle of the Handbrake page. Doing so ensures that the video will be compressed using web standards. It's near the bottom of the Handbrake window. You should see the following settings below the Video tab; if the settings do not match, you can change them by clicking a setting's value and then selecting the correct option in the drop-down menu:   Video Codec - This should be "H.264 (x264)".  Framerate (FPS) - This should be "30".  Peak Framerate or Peak - This box should be checked.  Encoder Level or Level - This should be "4.0". It's a green "Play" button at the top of the Handbrake window. Your video will begin compressing. On a Mac, you'll click Start here instead. This can take a long time, especially if your video is over 200 megabytes in size. Once your video finishes compressing, you'll be able to play it from its save location.

Summary:
Download and install Handbrake. Open Handbrake. Click Open Source. Click File. Select a video. Select a quality preset. Enter a file name. Check the "Web Optimized" box. Click the Video tab. Make sure that the settings here are correct. Click Start Encoding. Wait for your video to finish compressing.