You have two choices: slow and fast. Slow takes two days and stops at the village of Pak Bang overnight, where there are a few, what can be loosely termed, hotels. I did this.
Fast is one day and takes about 6 hours. If you decide on taking the slow boat, you will see the fast people wearing crash helmets occasionally speeding by in really small, noisy, aluminum boats. Some fast people will wave. The slow boat is better.
I spoke with someone who took the fast boat. Apparently, it is fast but painfully cramped and very bumpy. The two-day slow boat seems more romantic, but really, it is just cramped and slow. The only caveat is that the price is a lot less--about $15 versus $50 (depending on the exchange); and you are actually able to see the scenery.
The scenery is great. There is dense jungle on both sides of the river, an occasional mountain, and some very small thatched hut villages.
For the speedboats take a taxi (song thaew) to the landing south of the main market.
For the slow boats, go north to the end of the main street where there is a stone gateway. The launch area is behind the house here, on the right. There is also a good restaurant further off to the right, closer to the shore.
Do not miss the spiders on the way to the boat launch. Walking down the street to the boat landing, look up and check out the huge spiders and webs strung between the power lines that crisscross the road from time to time.
The boats leave in the morning, and I was told, only when there are enough people to fill it. This is not a problem, as far as I heard. Every traveler I met on this route got on the river when they wanted to. The boats hold about 30 passengers at the start. They pick up others at villages along the way, along with chickens, 50lb sacks of rice, tractor parts, gasoline, and assorted other freight.