1. Download and Install Google Chrome (if you haven't already for things like Adblock and the like.)
2. Download YouTube Downloader Chrome extension here. Click Keep to start download.
3. In Chrome, click the three bar menu button in top right corner, Go to Tools > Extensions.
4. Drag and drop downloaded file anywhere on extension window.
5. Click add to confirm installation.
6. Restart Chrome, go to youtube video you want to download, and see the new menu options. Enjoy!
EDIT- Made this a linked page up top.