To add a new episode to a TV show, look up the series title in the database, scroll through the main cast/credits, and click on the 'Add episode' button in the 'Contribute to this Page' section at the bottom of the page.
You'll be presented with a form that will let you enter the title, air date and number of the new episode. Fill in the required information, click on 'Continue' and follow the on-screen instructions.
After you've entered the new episode title, date, number etc., you'll see a section on the page called 'Recommended Information'. This allows you to add credits for the episode you're adding (e.g. the name of the director, writers etc.)
To add the names of the actors who starred in that episode (incl. guest appearances), select 'Add X credits' next to the 'Cast' option (where 'X' is the number of credits you want to add).
Please note that new episodes can only be defined by starting at the TV-series title pages using the process above and cannot be added by starting at the name page of any of the cast or crew.