The same way as if you add ‘serialized’ custom videos (e.g. courses) to Plex:
Use a library of the type ‘TV Shows’, but set its default agent to ‘Personal Media Shows’
Use the regular ‘tv show’ notation for file names and folder organization.
Use the mp4 file format, so you can add episode names and episode descriptions as embedded meta tags for Plex to read.
Add the series Poster, season posters and episode thumbnails (if you want) as ‘side-car’ jpg files.
https://support.plex.tv/articles/200220717-local-media-assets-tv-shows/
The only thing you need to add manually within Plex is the title and description of the “series” as a whole.
You can ‘stalk’ this user who asked recently several related questions about this
