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