yes that's about it, but honestly the article describes a wide variety of scenarios. Your specific case will depend on the type of application you wish to package, use the wiki as a reference until you are familiar with the process.
Best way to understand the construction of an extension is to deconstruct an extension, say one or two existing extensions in your optional directory using the command:
sudo unsquashfs -d extension_name extension_name.tcz
a perfect example of a simple extension showing structure and permissions, to start with is
sudo unsquashfs -d wget wget.tcz
permissions might take a while to understand so I'd follow the permission section from wiki guide to the letter
if you intend to maintain extensions then it's probably best to make an extension containing a script which automates the process, this is how I accomplish these repetitive tasks.
good luck