About half an hour before your bus is scheduled to leave (in my case it was at 20:30), you should be at the office at 327 Rambutri Road, รามบุตรี, Khwaeng Talat Yot, Khet Phra Nakhon, Krung Thep Maha Nakhon 10200, Thailand near Khao San Road. The Lomprayah office is in the curve of the street. In your booking confirmation email is also a map. This is their help hotline: +6626290011 and office hours are from 05:30- 22:00. Coming from the North you turn into this really busy street from the roundabout, between the 7/11 and the red sign of Swensen's. People with a lot of luggage may have some trouble getting through the night market. About 30 meters in, on the left side and right in the curve, is the Lomprayah office, slightly hidden behind a banner. Make sure to have your booking number on hand!
If you are on Khao San Road you have to find your way on the McDonald side of the road through the small alleys with all the shops to get to Rambutri Road.
At the office everything felt a bit rushed, one employee hands you your ticket and another a colored sticker with your seat number to put on your T-shirt and a color-coded sticker with the destination for your luggage. Make sure to tie that last one around a handle, it doesn't stick well. Then you sign in and someone will take your group to wait at the middle of the roundabout, behind the green sign, until the bus arrives.
The bus had only regular seats, so it is not a sleeper bus and the 8 hour drive will probably be a bit uncomfortable (especially if you were planning on actually getting some sleep). A travel pillow or a folded sarong helps a bit, and so do earplugs. The seats recline a little and blankets are provided. The bus is air-conditioned and has a toilet. I'm only 1.60 m, so I had plenty of leg space, but anyone taller than 1.90 m should expect some discomfort.
At 2:00 there was a 30 min break at a market with snack food. They also had a huge bathroom with clean toilets, but remember to bring toilet paper and flush with the bucket provided.
At 5:15 we arrived at the ferry pier, where the office was dark and still closed and we had to wait for almost 2 hours. There is a mini mart to the left, if you need anything while waiting, and on the opposite side is another array of clean toilets. The mini mart closes when the ferry opens for boarding, so do not plan on any last-minute shopping. When the info center on the right of the pier opens, you have to show your ticket for another sticker and to get a stamp. Around 7:00 people will start to line up for the "best" seats, but there was plenty of room for everyone, so I would not take it too seriously. After a short walk along the pier, you arrive at the boat. There are three seating areas: Downstairs (inside), in the middle (outside with sun roof) and on the top (outside without cover). I chose the middle and it was perfectly fine.
About 2,5 hours (of comparatively smooth sailing) later we arrived at Ko Nang Yuan to let some people on and off the boat and then turned towards the Lomprayah Pier on Ko Tao, where we arrived 5 minutes later.
From here on it is really easy: Get off the boat, grab your luggage and find your pick up or taxi at the end of the pier. Congrats, you have made it to Ko Tao! Enjoy your stay!