1. Need to find video and copy id.
YouTube will display an id (like HFv-4s1MSU8), when you save (or play) a video. You can use this id, and refer to your video in the HTML code.
2. Find the location where you want to embed the video on your site. Next, find the desired line in html
3. Add new tag (iframe) and setup attribute. Copy id(step 1) into URL's path
4. Result