This step is pretty self-explanatory, just find the Youtube video you would like to embed in your HTML file.
Now click the share button found near the bottom right of your video window.
After clicking the share button this window should pop-up, click the embed option.
After clicking the embed option, this window should pop-up.
Click the copy button on the bottom right of the window to copy the code.
If you followed these instructions correctly you should end up with a working YT video embedded in your HTML file