To publish your first podcast episode navigate to the Podcast area of your Castos dashboard.
If this is a brand new account then you likely won't have set up your first Podcast yet. This is the information about your show as a whole. That "meta" information like the Title, Description, Categories, and cover image that show up in all of the podcasting directories.
If you haven't set this up yet for your first episode you can do so by clicking the "Add New Podcast" button in the top right
If you've already got your main Podcast details set up you should see something like this
Click on the title of your podcast to view the list of Episodes there. If this is your firs time publishing episodes to that Podcast then that list will be blank
Then click 'Add New Episode"
This will bring up the Episode editor where you can fill in details such as the Title of your episode, the Description (i.e. Show Notes), and upload the audio file for that episode.
Below the media file section are some optional fields that you can fill in if you'd like. These are the iOS 11 podcast specific tags that Apple introduced mid-2017.
You can choose a featured image to go along with each episode you publish. This will need to be square, and 1400x1400px in size to be shown on iTunes.
Lastly, select the Publish Date and Time that you would like your episode to go live, and click Publish.
If you have already submitted your podcast RSS feed to iTunes (and it's been approved) your show will appear there within a few minutes.