Here's the breakdown:
* Riss: The "s" is pronounced like the "s" in "sin".
* wik: The "w" is pronounced like the "w" in "win", and the "i" is short, like the "i" in "bit".
It's important to note that "Riswyk" is an archaic spelling of "Ryswick", a town in the Netherlands. The modern Dutch spelling is "Rijswijk".