In order to install an application that is not on available on Google Play, you must first configure your device to download third-party applications.
Go to Settings > Security.
Next, go to Device Management and check the box next to Unknown sources.
Download and Install TubeMate
Download TubeMate to your Android tablet or phone.
Once the file is downloaded, open the notifications panel. Alternatively, you can go to Settings > Storage > Downloads.
Next, click on the tubemate_1-5.apk file. Tap on Install to begin the TubeMate installation.
Once installed, click Open to run the application.
Download Videos from YouTube
The TubeMate interface is similar to YouTube, but allows you to download videos. To do this, click on the magnifying glass located in the top right corner, and find the video yod like to download.
Play the video. To download the video, click the green arrow located at the bottom right of the playback window. In the window that opens, click the Download. Choose video format and then wait for the download to complete.
View Your Downloaded YouTube Videos
To view your downloaded videos, go to Settings > Storage > Photos /Videos.