Living in this part of Spain, it isn't always easy to find things, but after a few weeks I was able to collect everything I needed: square pots (ordered from Lithops.es), 1000 seeds (ordered from Cono's Paradise), and the quartz sand and vermiculita, as suggested by Khenai. Of these, the quartz sand was the most difficult to locate, but I ended up with a couple of bags of 2-3 mm. aquarium sand, produced by Edna, and a bag of larger-grade sand produced by Friskies:
Based on Khenai's instructions, I prepared a total of forty pots, but used three different mixes of sand and vermiculita, topping all the pots off with a white granite mixture of about 5 mm. I mixed the seed with some fine sand in an old herb shaker:
(ignore the contents of the shaker!) This allowed me to more easily spread the seeds across the forty pots -- time will show how well they were distributed!
Each pot was given a marker, indicating the mixture used and the date of sowing. Then, again following Khenai's instructions, I lightly sprayed the pots with water and covered each one with some cling-film. The final result:
So now it's wait and see!