I don’t think you have read thoroughly that article…
Just a small quote for you:
If you need to create a new file, it is a good idea to copy it from the next file in the hierarchy that exists. For instance, if you want a special display for Category 6, begin by copying the category.php file, or if you don’t have one, use archive.php, and so on.
Go step by step – you don’t really need any PHP, that’s just a cheap excuse for those that don’t want to do the work themselves. Put whatever html code you want ABOVE the Loop. Get familiar with the WP lingo and terms: template files, template tags, Loop. There are plenty of links from that article I linked above.
Category templates will always display ONLY the posts from your “specific” category, you don’t need any PHP wizardry…
We all learned how to work with themes by trial and error.