soo-DON-uh-muss
Here's a breakdown:
* soo like the beginning of "soup"
* DON like the name "Don"
* uh like the "u" in "cup"
* muss like the word "muss"
When referring to a pseudonymous name, it's pronounced with emphasis on the "name" part. For example:
* "They used a pseudonymous name to protect their identity."