No budget airlines fly there. So you fly AirAsia or Nok Air from Bangkok to Surat Thani, then from Surat Airport take either a minibus or big bus to Samui. You'll see the ticket counter as soon as you come out of the Baggagle Claim area. Minibus is 500 Baht and goes direct to the ferry pier. This is advertised quite prominently but if you want to save money there is the big bus for 350 Baht, which stops in Surat Thani town centre for half an hour or so to pick up other passengers. Prices include the ferry ticket.

Air Asia has the most convenient flight down to Surat, Nok AIr has the best timing for the return flight. ( I did this trip last month.)

Coming back you can get a big bus from Nathon on Samui to Surat Airport. I think the bus leaves around 10 or 10.30am.

thanks for your reply.

I'm guessing the minibus is the quicker option? How long does it take & how frequent are the ferries? I'm trying to gauge how long the trip from BKK to KS would take from start to finish

No idea how often the ferries run but I'd guess fairly frequently - every 30 minutes / 1 hour. Google should be able to help out with that. The minibus saves time as it goes direct to the pier and not via Surat Thanai town centre and also doesn't sop for half an hour there. In the minibus I guess it'd be around an hour to 75 minutes drive to the ferry pier then the ferry crossing. With the big bus it's around 2 hours drive.

Thanks for your reply, we've decided against KS on this occasion but it's good to have the info