Assigning icons

Hello!

I need help assigning a newly created icon to an app. Saving the icon from Icon-O-Matic and dropping it on the icon well of the FileType window of the app doesn't do the trick.
I also tried to export as HVIF and drag&drop that. No go. If Icon-O-Matic was writing the icon attributes correctly (Ticket pending), I could just drag&drop between icon wells...

Also, how is the "Add icon..." of the icon well context menu supposed to work? It opens Icon-O-Matic. There I create or load my icon. And then? Is the icon supposed to appear automatically in the icon well upon closing I-O-M?

What are the other Export options of I-O-M for?

I have so many questions... :)
Is there some I-O-M documentation anywhere?

Thanks a bunch!
Humdinger