Log in to WordPress dashboard. Under "Posts" click "Add New" to write new post or click "Posts" to choose an existing post.

When you're in your post, click "Add Video" icon. Select "From URL" tab. Now go and pick a video from YouTube. Once you've found one, simply copy it's link to "Video URL" and give a title for your video. Finally, click "Insert into Post" and YouTube video will be embeded in your post.