I just did this crossing in a 1997 Land Rover with a CDP. Solid current information was a little hard to find so I thought I would post this for the record.
The track to Diama was not obvious but on my GPS (openstreetmaps) and the locals told us it was the right one. There is a road being built and so the first stretch is very easy. It had rained recently so we were warned it wouldn't be passable in Nouakchott, but it was fine likely even for a non 4x4. We passed a loaded motorcycle driven by a frenchman coming the other way and he didn't have issues either.
I had read horror stories about this border, but it wasn't that bad. We had to pay various people along both sides of the border, normally 10 euros each. They didn't like Mauritanian money on the Senegal side but accepted it in the end.
There were only a few touts and they weren't obnoxious.
My CDP was time stamped for entry and I got a 48 hour passavant. They said I needed to go to Dakar before then to extend the passavant and my CDP properly stamped.
At this point a customs official (the chief of the post?) Moulay stepped in.
There were already two Spanish cars waiting and so we formed a small convoy to the Douane office in St Louis where letters were written, documents were photocopied, and 2 hours and 65 euros later we had passavants for 15 days. We got the impression he does this a few times a day. After he took us to the nearby "Camping Ocean" (where we assume he gets a kickback) for us to set up camp, have a few

s, and for him to ask for the money. Afterwards he went and got a large bowl of rice and fish for the group of us, and was generally a nice guy.
The total border crossing cost was around 100 euros, although I still need to go to Dakar (not within 48 hours) and get my carnet dealt with.
My friend had a newish Toyota and no CDP, but that's OK for cars less than 5 years old. He had to pay an extra 10 euros for the passavant and doesn't need to go to dakar.
A german we are traveling with had an older car and no CDP since he originally planned to go through Mali. That was a bit of a problem, but it got sorted at the same time for around 225 euros (150000 CFA). That price can vary depending on the mood of the official, your destination, and various other factors it seems.
We arrived at the border around 2:30 and left the St Louis Customs around 5:45, so a total of just over three hours.
I hope this helps somebody.
- Andrew