When you’re done editing your video for YouTube, simply drag and drop your intro into the timeline, make sure everything stays at its place and is not disrupted. And that is simply how you add an intro to your YouTube videos.
You can also add an intro video to your YouTube Channel by clicking on the manage channel or customize channel and clicking on the For New Visitors tab. Then, click on the Channel Trailer inside the YouTube frame and choose your suited intro video, the one you would want your visitors to see on top of your channel. Select that, and once you refresh your YouTube channel, you’ll be able to see that intro video on top!