Before I took this boat, I was reading different things on lots of different websites so I thought I would write a very clear account of my trip with up to date prices on here as quite a lot of people use this. Hopefully this will get rid of any worrying thoughts you might have.
So, we start in Chiang Rai bus station at about 11am. You jump on one of the red buses which is clearly marked Chiang Rai to Chiang Khong. No need to buy a ticket before, you pay on the bus when the journey is underway. Costs 65 baht and takes 2.5-3 hours.
You get dropped off in a side street next to a 7/11 and a bunch of tuk tuks. Grab some snacks, drinks or money from the ATM from the 7/11 and then get in a tuk tuk in your own time. You will be taken down to the pier where the passport control office is as well as the long tail boats over to Huay Xai. Tuk tuk ride is 30 baht each.
Once you've got your exit stamp in your passport which will take seconds as the only other people there will be the foreigners that were on your bus from Chiang Rai. Head down the ramp, buy a long tail boat ticket from the lady at the table on the right. Board the boat and 5 minutes later you are in Laos.
Go through passport control, get your visa and head up the slight hill to the T junction. On the hill there are a couple of agencies selling slow boat tickets and that's where I bought mine from. 240,000 Kip which includes free tuk tuk to the slow boat pier. A ticket at the pier costs 220,000 Kip but you then have to pay the tuk tuk yourself which will be 10,000 Kip. If you do go with the agency, when arriving at the slow boat pier, the tuk tuk driver will go into the ticket office with your passport and get the boat tickets and do the formalities for you while you get a drink or snacks. Very handy.
Obviously if you missed the slow boat that day then you'll have to stay in Hauy Xai. There are a few guesthouses but I recommend Sabaydee. To get there, when you reach the T junction St the top of the hill, turn left. The guesthouse will be on your left about 150m down the road.
When you are at the slow boat pier and have your ticket, it will say on it your number. Basically, everyone who is there when you are there will be on the same boat. The boat will be the one which is connected to the pier via a little wooden walkway. Its all very simple.
The boat is supposed to leave at around 11am but we left at 12. If you get a really early bus from Chiang Rai, youll be able to make the boat on the same day.
The boat journey. First tip. Get there early. The boat "leaves" at 11am. Get there at 10.00am-10.15am. You can buy sandwiches and snacks all over Huay Xai so dont worry about that. Don't sit right at the front, it might look good but just before the boat leaves they will probably rearrange things which might mean you getting a crappy seat at the back because its almost full. Instead, get one of the seats facing forwards, the first row if possible, more leg room. The seats aren't bolted down either so get ready for people pushing seats into your knees just so they can have a more comfortable journey. Once everyone settles down and the journey begins its very pleasant. The toilet on board is western but take toilet roll. As the boat moves, certain liquids end up everywhere as you can imagine.
Pakbeng! They say it takes 8 hours to Pakbeng. It took us 5.5 hours. It might be longer other days, I'm not sure. Pakbeng is a really nice, one road town/village. Pick a guesthouse but not the one the boatman offers you at the beginning. There are plenty to choose from and the same goes for restaurants. Just chill out in the evening, have a nice meal and a couple of beers.
Arrive early the next day as well. Maybe even get the same seats you had in the first leg or grab seats that you wished you'd had the previous day. You set sail again. This time, its supposed to be 8-9 hours. We did it in 7 hours. You may have read this bit already but the boat stops 10km outside of Lung Prabang at a "New Pier" which is made of really wet mud and old pieces of wood that make questionable steps. I was aware of this before the trip but most people on the boat weren't and were therefore very unhappy. We had a sit down protest for 30 minutes and would not let them take the bags off. We surrended when a driver from a hotel who was picking two girls up entered the boat and assured us this was official and its the way it is done now. After leaving the boat with your bag, you need to take a tuk tuk into town. On this occasion, you have to pay 20,000 Kip at a table where you get a ticket. You then give the ticket to one of the drivers and off you go. I'm guessing this ticket system is to stop you from just jumping from the tuk tuk when in the city and not paying. One couple told me they walked from the pier for a couple 100 meters and got a tuk tuk for 5000 Kip each. Worth noting!
So overall it is a very simple journey to take and very cheap considering how long you are on the move for. Do not worry about any aspect of it as it is a very well trodden route and you will be with many other travellers at all times throughout the trip (sometimes this bit is the worst)
Anyway, if you have any questions or concerns then please just reply.