Find your desired video on youtube or upload your video to youtube.
Click the "share" button underneath the vdieo, then click "embed".
Click the embed button again and then your code for the video embedding will be on the right.
Use the iframe tag to adjsut the width and height of your video thumbnail. Follow it up with the src tag then copy and paste everything within the quotations from the youtube code. (e.g. "https://www.youtube.com/embed/9YffrCViTVk")
Here is a sample video: