There is another, Google approved, method of watching YouTube videos while offline (kind of), and it doesn't involve a third party app. This also means you won't be using an app that violates YouTube's terms of service (though I think it's only the developer of the app who is violating the terms, not the user). The method is built right into the YouTube app. It doesn?t officially support true offline viewing (I think it's intended to work with a poor connection), though I have gotten it to work without WiFi.
1. Open the YouTube app while online.
2. Go to Menu (3 vertical dots in the upper right corner) > Settings > Preloading.
3. Place a checkmark in the "Preload watch later" box.
4. Choose a video to watch. It will automatically start playing, but you can pause it if you want to.
5. Tap the + icon (a plus sign within a small, white box) that is located above the video. The "Add to" window will pop up.
6. Tap the "Watch later" option. A notification will appear, telling you the video has been added to your watch later playlist.
7. At this point, you can continue watching the video or watch a different video, or, I think, close the YouTube app. The video you chose to watch later will be downloaded in the background and saved to some obscure location on your device. Unfortunately, there's no way to tell when the download has completed, though the activity indicator on your WiFi icon might give you a small clue. The video can only be played back by the YouTube app.
So, that's how you download (preload) it. To watch it, you're supposed still have some sort of internet connection, even a very weak one. This is because the app needs to have you signed into your account, I think. But what I've found is that if you don't exit the YouTube app before going offline, you should be able to play the videos you preloaded, even after you're offline (maybe, possibly, perhaps). I've gotten this to work, but it's kind of funky.
1. Swipe the screen to the right to reveal your account and channels panel. Under Account, tap your account name. Your account page will appear.
2. Tap the "Watch later" box. The "Watch later" page will appear.
3. You'll see a list of all the videos you marked as "watch later". Those videos which you also preloaded should have a green download arrow in the lower right corner of their shortcut box. While online, tap a video in the list, let it play for a few seconds, then go back to the list and do the same for the other videos. Don't exit the YouTube app.
4. Later, when you are offline, you should be able to go to the "Watch later" page and watch each video that you preloaded, as long as the green download arrow icon is showing next to the video (maybe, possibly, perhaps).
5. To remove the video from your "watch later" playlist, go to the "Watch later" list. Tap the download arrow, then tap the "Remove" option that pops up.
Or download one of those YouTube downloader apps. They work really well!