First find your clip on YouTube. It may be one you've uploaded for the purpose or one that you'd like to recommend to people by including it in what you're writing. When you've found your clip on YouTube, copy its url from the box they supply or from the address line of your browser when you are viewing it.

Now click on the video icon on the left-hand side of the New Post screen.

Image by OneClimate

The attribution and tags will be automatically be taken from whatever is on YouTube. You can add a short description to help other users identify the clip in the future.

Then click on 'insert' at the bottom of the screen and the video will be embedded in your page. Then we the page is displayed, people will be able to simply click on the video to play it right there on the page.