The simplest way to install Ktube is via Ubuntu Software Center.
Open Ubuntu Software Center and search on Ktube:
Click on Ktube Media Downloader to start the install process.
Click on install and Ubuntu will start the install process. It will ask for your password and once entered it will start.
You can also install Ktube via Snap from terminal. Open the terminal and type:
sudo snap install ktube-media-downloader
Once installed you can download Youtube videos.
If you are not a big fan of snap applications you can install via .deb file as well.
Download the application here.
Once downloaded you can go to the directory and click on the downloaded file and follow the screen.