Laos (Time to process: 30 minutes)
Firstly, immigration. There are two landing areas on the Mekong: one handles passenger boats and the other cargo/vehicles. The latter vehicle landing does NOT have immigration facilities. You have to go to the passenger landing area.
This is located in the middle of the one-way section of the water-front road, down a narrow side street. Just present your passport, and you are processed.
You then walk back into Laos and back the 1km to the vehicle landing area!
At the vehicle landing, you need to pay 200Baht at the barrier. This charge may depend on your vehicle - remember that I have a trailer, so this charge may be less for a vehicle-alone.
The Customs office is on the left as you go down the sloping jetty road. Enter the office, and present your carnet. Everything is processed without any problems.
This completed, just roll down the sloping landing jetty road, and wait for the ferry boat to arrive. This shuttles between Thailand and Laos on an as-full basis. The boat is Thai-owned. The cost for the crossing (with trailer) is 1000Baht. For a full truck (Thai-registered) I noted that over 2000Baht was handed over, so, although there were no listed charges, I assume I was not charged too much more than a local.
Doing this crossing the other way, I expect that you'd be free to wander into Laos to process your immigration formalities "on trust". How they stop you just driving into Laos and never being seen again, I don't know.
Thailand (Time to process: 30 minutes)
When landed, park on the jetty. Go to the Immigration office on the right hand side at the top of the jetty-road. Get a visa (free, 30 day,extendable), and then call at the customs office which is the next one back down the jetty-road.
Here, if you are like me, you will be told to take your Carnet to the central Customs Headquarters just off the main street in the town.
You can now either disappear into Thailand, or be honest and go to the Customs Headquarters! If you choose the latter, you'll find the office down a side road on the left, after the Post Office. It is sign-posted by a small, brown arrowed-sign, partly obscured by a shop billboard. So keep your eyes open for the sign on the left.
The boss will be called, who will complete all the necessary Carnet paperwork. Note that the Thai-customs entry paperwork (computer printout) is not issued here. I hope this will not cause any problems on exiting Thailand later.