I get many requests for information on how to create karaoke CD+G discs from YouTube karaoke videos. The fact is that while many of the karaoke videos were created from CD+G song tracks the process is not reversible. In short, you cannot create a CD+G disc from YouTube videos.
This doesn't mean that you cannot play the videos on your karaoke machine. Fortunately, many modern karaoke machines support playing various file types from an SD card and/or a USB thumb drive. These file types virtually always include AVI or MPEG and often both.
What you will need to do is download the YouTube karaoke videos to your computer's hard drive, convert them to a supported format, and transfer them to an SD card or USB thumb drive. A program like YouTube Downloader makes the process very easy. Just drag and drop or copy and paste the URL for the video you want to download, choose the output video format, select the output folder (or device), and click the download button.
As a bonus this program will also covert the videos into a format that will play in your iPod, iPhone, and other mobile devices.