Creating an entry in the Codex is as simple as typing the name of the page you wish to create into the search bar and clicking Edit, the last button in the group. You can, alternatively, navigate to this page. (Pay no attention to the “external” link designator here, the page is local.)
Once you’ve named the page you’re creating, you’re free to fill it with content. If you wish, you may later decide to add it to an existing Structure for easier searching/navigation to related pages. (e.g. Characters/Register of Citizens and the Imperial Atlas, above).
Upon creation, you may also wish to add Tags for quick classification & searching, as well as an Edit Description detailing anything specific about the page creation moment.
Note
It’s generally not necessary to leave an Edit Description for page creation. TikiWiki is pretty good about being specific about page creation events in the edit log.