Go to the immigration office near the airport in NST and fill in the form and pay your money. I usually copy every page with stamps on it in your passport. There is no public transport to this office so you may need a Grab or motorbike to take you there.
Somebody somewhere says something in passing and the media pick up on it and run with it. Thailand is well known for throwing things like the ETA or in this case the TM6 out for public comment then nothing happens. If the 1st of May comes and a TM6 has to be done OK we will have to do it. I see this as a replacement for the ETA and Im not surprised they haven't tacked on the 300Baht entrance fee. For those who live in Thailand and regularly travel it maybe a problem however most countries you enter are going some sort of Electronics ETA.