Kevin has a bunch of tutorial on his website. Go to Tutorials tab, under HTML, you can find his Jekyll Tutorial.
Remember to add a dot (.) just before the / to indicate parent folder instead of root. For instance:
<link rel="stylesheet" type="text/css" href="./styles.css">
<a href="./index.html">Home</a>
<a href="./contact.html">Contact</a>
<a href="./beach.html">Beach</a>24:34 - Creating Beach.html
26:11 - Creating folder images
28:03 - Update css as file not found when rendering beach.html due to relative path. Note: here Kevin use slash to go to "root" folder, but I have to use ./ to go to current folder instead.
30:10 - Add permalink to Beach.html
30:40 - update permalink using Placeholders (i.e. from beach to :title)
32:30 - create _config.yml file and move the permalink
35:10 - create hike.html