Step 1: Write a framework and create your files.
Writing your framework for your code before you do anything else is important. If you know what video you want, but don't know how to style the page, you need to style the page before anything else. Creating the framework around your video placement is okay.
Step 2: Find a video to imbed.
For this example, I will be using a EmKay video, on the subreddit of r/technicallythetruth, a subreddit of examples for technically true things.
Step 3: Create/Edit line of code to embed video.
The main way to embed a video from an external source (a link) in HTML5 is using the [iframe] tag. All this does is access a file from another database, and load it. Most video sites like YouTube will have pregenerated code. This is useful, but do not be too reliant on this code as sometimes you're using the [video] tag for downloaded files on the same data storage system.
Step 4: Load the page, enjoy the video!
There needs no explaination, once the video code line is written, edited, and functional, the video embed is all set and available to watch!