By bus is the easiest way. I take a bus from Ekkamai bus station
Buses run every 30 minutes or so. I get a bus to Pattaya and from there i take a ferry to the island.
The bus from BKK to Pattaya costs about $4 and takes less than 2 hours.
The boat costs around $3 and takes around 20-40 mins depending on the waves.