It's a mouthful, and it's best broken down into parts to make it easier to pronounce:
1. "llanfairpwll": This is pronounced like "lan-vair-pool"
2. "gwyngyll": This is pronounced like "gwin-gil"
3. "gogerychwyrndrobwllllantysiliogogogoch": This is the tricky part, but it's essentially a series of sounds like "go-ger-i-chwi-rn-dro-bu-ll-lan-ti-si-li-o-go-go-goch".
Putting it all together, a good pronunciation would be:
"lan-vair-pool-gwin-gil-go-ger-i-chwi-rn-dro-bu-ll-lan-ti-si-li-o-go-go-goch"
It takes practice, but with enough repetition, you can master this tongue twister!