* MAY-sid-er-wen
Here's a breakdown:
* MAY: The "a" is pronounced as in the English word "may"
* sid: The "s" is pronounced like the "s" in "sin"
* er: The "e" is pronounced like the "e" in "her"
* wen: The "w" is pronounced as in "wet" and the "en" is pronounced like the "en" in "pen"
It's a bit of a mouthful, but with a little practice you'll be able to say it like a native Welsh speaker!