To publish your first podcast episode navigate to the Episodes are of your Castos account.
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.