On behalf of the Society of Indexers I am the originator of and tutor for several online workshops.
For indexers at all levels:
How to edit an index: creating order out of chaos: newly revised and updated
Writing index entries: finding the words: newly revised and updated
and coming soon: Indexing the past
For editors:
Indexing for editors: new in 2020
The Society of Indexers offers other online workshops too: click here for details.