To insert Youtube video into WordPress Post/Page/Widgets, just follow these steps:
WordPress 4.x (4.9, 4.8,…)
Method 1. Paste the Youtube video link
With this method, you can’t change width and height of the video
Step 1. Writing a new post (or page) or edit an existing one
Step 2. You need to click on a line where you would like to insert a Youtube video and click on Add Media
Next
and this is the result
Method 2. Paste embed code
Step 1. Click on Share
Next, click on Embed
Click on Embed code and Copy
Step 2. Click on a line where you would like to insert Youtube video, then click Text
and paste the embed code
You can change the width and height of the video by changing the number in width and height.
WordPress 5.x (5.0, 5.1,..)
Insert/Embed Youtube video into WordPress Post or Page
Edit Post or Page > Click (+) icon > Video
Click Insert from URL
Paste Youtube Video Link
Insert/Embed Youtube video into WordPress Widgets
Appearance > Widgets > Drag Video widget
Next, Insert from URL > Paste Youtube Link > Add to Widget
Save