It's pronounced approximately like this:
hlan-vire-poo-ling-guh-win-guh-gill-go-ger-u-churn-dro-boo-th-llan-sili-o-go-go-goch
Here's a breakdown:
* Llanfair: "hlan-vire"
* pwll: "poo-ling"
* gwyngyll: "guh-win-guh-gill"
* gogerychwrndrobwllllansiliogogogoch: "go-ger-u-churn-dro-boo-th-llan-sili-o-go-go-goch"
It's a mouthful, but with practice, you can master it!