SoupraTours runs buses from Moroccan Dakhla to Mauritanian Nouadhibou and Nouakchott. But, unfortunately, not every day. Every day the bus goes from Dakhla to the border Gergerat (at 9 am, 190 dirhams = 18 €). From there, after the Moroccan border control, you need to take a taxi to drive 4 km of no man's land to the Mauritanian control.
The bus traveled 400 km from Dakhla to Gergerat in 5 hours. We arrived at the border post at 14:15, but for another 45 minutes the border was locked: the border guards were having lunch. The stamp on the Moroccan border was easily placed. We traveled 4 km of no man's land in a taxi. The taxi driver charged 100 dirhams (about 10 euros) per person for taking us to Nouadhibou. At 16:00 we reached the Mauritanian border post and sat down to wait for visas. At 16:25 I received a fresh visa with a stamp. No documents were needed, only a passport and 55 euros.
50 km from Nouadhibou, I pushed the car once and spent two of the fifteen copies of the passport prepared in advance at checkpoints. At 18:00 arrived in Nouadhibou. Everything went pretty smoothly. The car of patience, which I took with me to Africa until I needed it.