Get the link of the YouTube video and visit y2mate , paste the link of the video and you'll see a download option. However, it won't really be at the "highest quality". So if you want the highest quality, obviously taking up bigger space, visit i don't really know the name of the site , paste the link and click convert/download.
Warning: these sites normally open up different tabs(for ad purposes). So be sure to always close them and go to the original tab